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Connecting Rod-to-Bushing Interference Fit


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I'm hoping someone can tell me what the fit is between the connecting rod and the bronze bushing on the small end...in other words, how much interference there is supposed to be.  My machinist can't find the spec in any of his books and I can't find it in any of mine.  My engine is a 265, but I believe all the Chrysler/Dodge flatheads use the same wrist pin bushings, so I don't think it matters much exactly which engine we're talking about. 

 

The bore in one rod is damaged, so he needs to install an oversize bushing and needs to know the proper interference, meaning he can't just push one bushing out and push another one in its place.  He can get an idea of the fit by measuring the inner diameters of the bores from the other rods, but he would prefer to find a written spec.  If no one has that, then maybe one of you can find out what your machinist did?

 

I don't think it matters much if you installed a new standard size bushing.  The amount of interference should be about the same for a standard size or oversize bushing.

 

Many thanks.

Edited by Matt Wilson
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.0001" to .0004" spec for a 1953 Dodge 2-1/2 ton 265 same as all 25" engines....

IMG_5121.JPG

IMG_5120.JPG

Edited by Dodgeb4ya
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2 minutes ago, Matt Wilson said:

Is that the fit between the bushing and the bare rod, or between the pin and the bushing?  Sounds like the latter.

That's the fit between the piston pin and rod bushing

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I hate to say this but if your machinist does not know how tight the bushing fit is in the rod... well....

He is not well heeled in connecting rod re- conditioning.

There are plenty of sources on this press fit info available to him.

I found these general sources... he must have access to a commercial engine re- builders services  or another machine shop that can help him out.

I have never seen in any MoPar shop manual this press fit data. It's known among the machine shop old timers I'm sure.

https://www.practicalmachinist.com/vb/general-archive/bushing-press-fit-93752/

https://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=farmall&th=960505

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I’ve measured clearance a ton of times but for most of the hotrods and rebuilds I messed with new high quality rods were cheap insurance. So they came already bushed for piston pins.

 

in my opinion there is nothing wrong with doing what you’re saying on an old forklift motor- it’s not going to throw off the balance at 12,000 rpm! My 230 almost never saw north of 3500. Besides what’s cooler than making the stuff you already have keep working!

 

By this chart I google-fu’d you’d probably be safe inbetween 1/2 a thou and 2 which sounds right to me? I know more like .003 and you’ll have a hell of a time trying to get it in there.

 

Chart:

Class V Locational Interferance Fit Tolerance Chart for Holes and Bolts per McDonald Douglas Design Guide "Machining Tolerances".

Units given in inches

Locational interference fits are used where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirements for bore pressure. Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit. 

Class V Locational Interference Tolerance Chart for Holes and Bolts
Basic
Diameter
Hole
Diameter
Shaft
Diameter
Interference
Maximum
Minimum
.125 .1240
.1248
.1254
.1250
.0014
.0002
.156 .1562
.1570
.1576
.1572
.0014
.0002
.188 .1875
.1883
.1889
.1885
.0014
.0002
.213 .2188
.2196
.2202
.2198
.0014
.0002
.250 .2500
.2508
.2514
.2510
.0014
.0002
.281 .2812
.2820
.2828
.2823
.0016
.0003
.312 .3125
.3133
.3141
.3136
.0016
.0003
.344 .3438
.3446
.3454
.3449
.0016
.0003
.375 .3750
.3758
.3766
.3761
.0016
.0003
.406 .4062
.4070
.4078
.4073
.0016
.0003
.438 .4375
.4383
.4391
.4386
.0016
.0003
.469 .4688
.4796
.4704
.4699
.0016
.0003
.500 .5000
.5008
.5018
.5011
.0016
.0003
.562 .5625
.5634
.5643
.5637
.0018
.0003
.625 .6250
.6259
.6268
.6262
.0018
.0003
.688 .6875
.6884
.6893
.6887
.0018
.0003
.750 .7500
.7509
.7518
.7512
.0018
.0003
.812 .8125
.8134
.8145
.8138
.0020
.0004
.875 .8750
.8759
.8770
.8763
.0020
.0004
.938 .9375
.9384
.9395
.9388
.0020
.0004
1.000 1.0000
1.0009
1.0020
1.0013
.0020
.0004
1.125 1.1250
1.1260
1.1273
1.1265
.0023
.0005
1.250 1.2500
1.2510
1.2523
1.2515
.0023
.0005
1.375 1.3750
1.3760
1.3773
1.3765
.0023
.0005
1.500 1.5000
1.5010
1.5023
1.5015
.0023
.0005
1.625 1.6250
1.6260
1.6275
1.6266
.0025
Edited by Radarsonwheels
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I cannot cite the page number, but the Machinery's Handbook states that the OD of a bushing should be around .001 over the nominal size, and the hole into which it is being pressed should be .001 under the nominal size.  That is the approach I used when installing oilite bushings (sintered bronze) in my door hinges.  (Obviously there is a big difference in use between hinges, and engine bearings - not denying that.)

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14 hours ago, Dodgeb4ya said:

I hate to say this but if your machinist does not know how tight the bushing fit is in the rod... well....

He is not well heeled in connecting rod re- conditioning.

There are plenty of sources on this press fit info available to him.

I found these general sources... he must have access to a commercial engine re- builders services  or another machine shop that can help him out.

I have never seen in any MoPar shop manual this press fit data. It's known among the machine shop old timers I'm sure.

https://www.practicalmachinist.com/vb/general-archive/bushing-press-fit-93752/

https://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=farmall&th=960505

Yes, believe me, the thought crossed my mind too, regarding whether the machinist is very knowledgeable in this type of thing.  If I don't find a number anywhere, then I'm going to ask him what's used for other engines.  There are enough other engines that use bushings in their rods that it seems he ought to be able to get a good number. 

 

Interestingly, he is an old-timer, probably in his 70s, so I'm a bit surprised he doesn't already know the answer to this.  When I asked him about clearance between the pins and the bushings, he said he goes by feel because he's done so many of them.  If he's done so many, why is there a question about interference between bushing and rod?  I suppose most engines don't need an oversize bushing...just replace the old bushing with a new one and he's never needed to know the fit dimensions.

 

I appreciate the link to the article.  I had found that one as well.  If needed, I'll relay that info to him.

 

Another point of interest....I called another machinist who I'm told does a lot of work on our flatheads (out of state guy) and asked him this question, and he didn't know either.

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On 2/8/2019 at 10:24 PM, Radarsonwheels said:

I’ve measured clearance a ton of times but for most of the hotrods and rebuilds I messed with new high quality rods were cheap insurance. So they came already bushed for piston pins.

 

in my opinion there is nothing wrong with doing what you’re saying on an old forklift motor- it’s not going to throw off the balance at 12,000 rpm! My 230 almost never saw north of 3500. Besides what’s cooler than making the stuff you already have keep working!

 

By this chart I google-fu’d you’d probably be safe inbetween 1/2 a thou and 2 which sounds right to me? I know more like .003 and you’ll have a hell of a time trying to get it in there.

 

Chart:

Class V Locational Interferance Fit Tolerance Chart for Holes and Bolts per McDonald Douglas Design Guide "Machining Tolerances".

Units given in inches

Locational interference fits are used where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirements for bore pressure. Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit.

Thanks, I had run across this chart too.  Looks like it's for solid shafts installed into bores.  I imagine the fit can be tighter in such a setup than with a thin-walled bronze bushing, but it's a place to start.  I'm going to poke around to see if I can find an engineering page that will give me some idea of interference for a thin-walled bronze bushing.  Like you, I tend to think a half-thou to over a thou would be pretty good.

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17 hours ago, Eneto-55 said:

I cannot cite the page number, but the Machinery's Handbook states that the OD of a bushing should be around .001 over the nominal size, and the hole into which it is being pressed should be .001 under the nominal size.  That is the approach I used when installing oilite bushings (sintered bronze) in my door hinges.  (Obviously there is a big difference in use between hinges, and engine bearings - not denying that.)

So Machinery's Handbook is recommending a total interference of 0.002"?  What diameter and wall thickness of bushing is this for?

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9 hours ago, Matt Wilson said:

So Machinery's Handbook is recommending a total interference of 0.002"?  What diameter and wall thickness of bushing is this for?

Material type would be a factor as well.

 

Have you measured the outside diameter of new, unused conn rod small end bushings and compared that dimension to the inside diameter of the small end of a known good bare rod, i.e. one with the old bushing pressed out?  That difference will give you some idea of the interference fit of a standard rod-to-bushing.

 

I'm assuming that you cannot replace the rod with the oversized small end inside diameter?

 

Is he going to burnish the inside diameter of the new bushing to size them and seat the outside diameter into the rod small end after the bushings are pressed in?

Edited by Elwood
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2 hours ago, Elwood said:

Material type would be a factor as well.

 

Have you measured the outside diameter of new, unused conn rod small end bushings and compared that dimension to the inside diameter of the small end of a known good bare rod, i.e. one with the old bushing pressed out?  That difference will give you some idea of the interference fit of a standard rod-to-bushing.

 

I'm assuming that you cannot replace the rod with the oversized small end inside diameter?

 

Is he going to burnish the inside diameter of the new bushing to size them and seat the outside diameter into the rod small end after the bushings are pressed in?

Yes, you're absolutely right - bushing material makes a difference, as does the parent part material (in this case, a steel rod).  I've seen Mopar flathead bushings that were solid brass (or maybe they were bronze) and others that were brass (or bronze) on the inside, with a steel backing on the outside.

 

As for measuring the inside diameter of a good rod with no bushing, I did that just last night with a couple of 251 rods that I believe use the same bushings as my 265.  I removed the bushings from two rods (turns out they were steel-backed brass/bronze split bushings) and used telescoping gauges and outside micrometers, arriving at inside diameters ranging from 0.9135 - 0.9138" on one rod, and 0.9140 - 0.9142" on the other rod.  I verified these measurements with a set of calipers, which don't measure down to the ten thousandth of an inch, but still provided numbers that gave me confidence in my readings.  What's interesting is that the standard size bushings I assume were in those rods are advertised as having an OD of 0.9135", implying that there would be no interference fit and could actually be a little loose.  The standard size bushings I have in my possession are all of the split variety, so getting a good diameter reading is difficult.

 

As for burnishing, I am planning to ask him about that.  I've read that some engines require burnishing of rod bushings.  The couple of rods whose bushings I removed last night have very visible and "feelable" machining marks, meaning circular grooves along the entire length of the bore, and the bushings also have those same marks that can be seen and felt.  Makes me wonder if those bushings were burnished, especially when considering the apparent loose fit of a new bushing in the bore per my measurements above.  I'm pretty certain these rods have never been reworked, so I think all this is factory original.  Conversely, I read in one manual that a new bushing can be used to push out the old bushing, thereby accomplishing the removal of the old and installation of the new at the same time.  There was no mention of burnishing that I recall.

 

EDIT: I rechecked all my manuals just now, and none of them mention anything about burnishing.  Some don't say much at all about installation of new bushings, and others just say to install and then use a hone or diamond boring tool to achieve the finish.  Still gotta wonder if burnishing would be best, though (followed by honing).

Edited by Matt Wilson
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Burnishing the bushing is important... locking it in place.

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14 hours ago, Matt Wilson said:

So Machinery's Handbook is recommending a total interference of 0.002"?  What diameter and wall thickness of bushing is this for?

It's a thick book, so unfortunately I doubt I could easily find it (to get the exact context in which the statement was made).  But in relation to main bearings, I know that back in the late 50's - early 60's, when my dad worked at a Chrysler-Plymouth-DeSoto-Dodge dealership in Tulsa, they normally rebuilt engines so tight the starter couldn't turn them over.  They literally drug the car around the block (down-town Tulsa) a few times to loosen it up before attempting to start it with the starter.  I guess this was their way to 'burnish' the bearings after the rebuild.....

Edited by Eneto-55
correct spelling
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4 hours ago, Matt Wilson said:

Yes, you're absolutely right - bushing material makes a difference, as does the parent part material (in this case, a steel rod).  I've seen Mopar flathead bushings that were solid brass (or maybe they were bronze) and others that were brass (or bronze) on the inside, with a steel backing on the outside.

 

As for measuring the inside diameter of a good rod with no bushing, I did that just last night with a couple of 251 rods that I believe use the same bushings as my 265.  I removed the bushings from two rods (turns out they were steel-backed brass/bronze split bushings) and used telescoping gauges and outside micrometers, arriving at inside diameters ranging from 0.9135 - 0.9138" on one rod, and 0.9140 - 0.9142" on the other rod.  I verified these measurements with a set of calipers, which don't measure down to the ten thousandth of an inch, but still provided numbers that gave me confidence in my readings.  What's interesting is that the standard size bushings I assume were in those rods are advertised as having an OD of 0.9135", implying that there would be no interference fit and could actually be a little loose.  The standard size bushings I have in my possession are all of the split variety, so getting a good diameter reading is difficult.

 

As for burnishing, I am planning to ask him about that.  I've read that some engines require burnishing of rod bushings.  The couple of rods whose bushings I removed last night have very visible and "feelable" machining marks, meaning circular grooves along the entire length of the bore, and the bushings also have those same marks that can be seen and felt.  Makes me wonder if those bushings were burnished, especially when considering the apparent loose fit of a new bushing in the bore per my measurements above.  I'm pretty certain these rods have never been reworked, so I think all this is factory original.  Conversely, I read in one manual that a new bushing can be used to push out the old bushing, thereby accomplishing the removal of the old and installation of the new at the same time.  There was no mention of burnishing that I recall.

 

EDIT: I rechecked all my manuals just now, and none of them mention anything about burnishing.  Some don't say much at all about installation of new bushings, and others just say to install and then use a hone or diamond boring tool to achieve the finish.  Still gotta wonder if burnishing would be best, though (followed by honing).

 

I suppose you could use the new bushing to push out the old, but I've always used a bushing driver or arbor to do the job.  Using the new bushing seems to me to be an invitation to deform the leading and/or trailing edges of the new bushing if the old one puts up any resistance, or if it's worn thin enough that the new bushing doesn't get a good bite on it.

 

I can't recall now where I read about people having problems with the small end bushings working loose, which suggests that the interference fit was not interfering enough.  Burnishing should correct that problem.  Some manufacturers, such as Packard, required that the pin bushings be burnished after installation, but as you noted, I've never been able to find any reference to the process in a Mopar manual.  But burnishing should do exactly as suggested: form the softer OD of the bushing to the imperfect but harder ID of the rod small end, thereby locking the bushing to the rod.

 

I have a Cogsdill Roll-a-Finish tool (I think it's a 3099070 D-13 SR-844) with an MT2 shank that I fit up in a floating reamer holder to burnish the pin bushings after pressing them in.  Good info on the Cogsdill web site about the process and what it does.  A nice feature of this tool is that it's adjustable over a small ID range to fit the bushing.

Edited by Elwood
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Got an update. I called a couple of other machinists and they couldn't tell me how much interference there should be. I even found a copy of a 2006 edition of the AERA connecting rod manual, and while it provides this kind of info for some engines, it doesn't for our flatheads. I then called Sealed Power yesterday and they told me the bushings need to be installed with about 0.004 - 0.0045" of press fit. I thought the rep didn't know what he was talking about and so did my machinist. That sounds like a ludicrous amount of interference. So I dropped into another machine shop - a big outfit - and talked to their head machinist, who has been doing engine machine work for 40 years, and he said that was about right for bronze bushings, which is what these are. I thought they were going to be steel-backed bronze, based on the description on NAPA's website, but they are bronze all the way through. I know this because I got a hold of three different parts (three different part numbers - one standard outside diameter and two different oversizes) and looked at the parts myself, and they are bronze. Anyway, the machinist said if the interference is only 0.0015" or so, which is what I was thinking, then the bushings will "fall" out. He also said burnishing them to expand them out against the rod bore is the best way to do it, but not many shops have the setup to do this, but he does. He also said such a large press fit is not needed for steel bushings, nor is burnishing. I got the impression from him that steel bushings only need about 0.002" interference, bit I'm not sure how accurate that is.

Further confirmation of the tight press fit for bronze bushings is the fact that when I measured the three different bushings I got a hold of, the ODs were all 0.004 - 0.005" larger than advertised online (again by NAPA as well as other sources). I guess when the OD is stated, they mean the final installed OD, which is essentially the size of the rod ID, but that was not clear from the specs they provided. I think I'm starting to understand how this works, though.

Since the machinist I spoke to this morning really seemed to know what he was talking about, I went and picked up my rods from my previous machinist and am planning to take them to this new guy.

I'm posting this in hopes it will help someone else at some point, maybe even me

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The original MoPar bushings where steel backed.

This second machinist sounds like he knows his stuff!

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1 hour ago, Dodgeb4ya said:

The original MoPar bushings where steel backed.

This second machinist sounds like he knows his stuff!

Yes, I pulled the bushings out of a couple of 251 rods that I believe have never been reworked and they were steel-backed.  They even had the DPCD logo on the back.

 

Interestingly, I found a Hudson flathead six  rebuild page online, and it talks about using steel-backed bushings, but then goes on to say they should be burnished in place using a particular burnishing machine.  It looks like it's an original factory set of instructions.  This seems to contradict what my new machinist told me the other morning.  Oh well, in any case, my bronze bushings will be getting burnished, and that's what matters to me.

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On 2/16/2019 at 6:37 PM, Elwood said:

Matt, do you have a link to that Hudson rebuild page?  I'd like to read that discussion.  Thanks!

Sure, here you go!

 

www.hudsonjet.hetclub.org/hudsonjetengine.html

 

It's interesting to note that this page says the bushings should be pressed out, but my previous machinist said this can often lead to problems because the thin-walled bushings most engines use can easily become cocked in their bores and cause damage on the way out.  He thinks that's what happened to the one rod of mine.  He prefers to use a hacksaw blade and carefully slice through the bushing and then it is easy to remove.  I did that with the remaining bushings after I picked up the rods from him.  It was a little work, but not bad.  I did leave small hacksaw marks in the the bores of the rods, but they are small enough that I'm not worried.

 

In the one rod bore that was damaged, I took measurements using a telescoping gage and an outside mic,  and the damaged area appears to be about 0.010" oversize or more (0.9326" in the damaged area vs about 0.9314" in the undamaged regions).  The damage covers about half the length of the bore and about half of the circumference of that portion of the bore.  It looks like even the oversize bushings won't be large enough to make up for that much damage, as they are only intended for a 0.0055" oversize.  I'm guessing my new machinist will have to try to find a bushing from a different application to adapt to that rod.  I hope he's able to find such a thing.  If not, then I'll have to go hunting for a replacement 265 rod, and they are not easy to find.

Edited by Matt Wilson
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Thanks for that link, Matt.  That Hudson bearing burnishing tool looks a lot like the Packard one.  It's almost a broaching operation.  I like the roller approach instead, and the tool is probably a lot easier to find nowadays than the factory tool!

 

I can understand his concern about damaging the bore, especially when removing a well worn bearing.  That's why I mentioned using an arbor above.  Your machinist should be able to make one up on his lathe without too much time or trouble if he doesn't already have one at hand.  Unless the pin bore is worn cocked, the arbor should push the bearing straight out of the rod.

 

If that saw blade mark is on the side of the rod ID, you'll be less likely to have a problem, but if it's top or bottom (where most of the force is), well...these are thin-wall bearings, and it might burnish itself into that recess through use, transferring the imperfection to the ID of the bearing.

 

Are you using steel-backed bearings?  If you aren't set on those, you can source solid bronze bearings of standard and oversize dimensions through places like McMaster-Carr, and then turn the OD to the necessary size for that damaged rod (assuming, of course, that the rod bore has been reamed or honed true again first).  Then ream the ID to a near undersize, burnish the bearing, and finish to final ID.  Again more work, but perhaps easier than finding a 265 rod that's usable.

Edited by Elwood
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The new machinist said he uses a ball-type burnisher.

 

The saw blade marks are at the bottom of the bore, where the force is large, but they are so narrow that I'm not concerned.  Also, they don't go the whole length of the bore.  Rather, they are restricted to the ends of the bores.  At least half of the bore length (probably more) does not have any saw markings.  I purposely avoided making cuts along the sides of the bores  because this is where the metal sees the highest tensile stress when the piston reaches TDC on the exhaust stroke, and I see saw cuts as the perfect way to introduce stress risers that can produce fatigue cracks.  My previous machinist had also made his cut on the bottom of the bore,  although he is a bit more skilled than I, so his cuts are a bit shallower.

 

The bushings I can purchase from Sealed Power are all solid bronze with no steel backing.  This is the case whether I purchase standard sized bushings or oversize ones (despite advertisements that say they are steel-backed).  I'm not set on steel-backed bushings at all.  In fact, I like the idea of using the bronze ones because my new machinist says he'll burnish them, which he won't do if they are steel-backed.  I'll just have to consult with him about options for getting a bushing that will fit that one rod, or even making one.

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