218 dodge rod bolt diameter issue

[dhughens]

  I am doing a project with a Pontiac 268 straight8 using Dodge 218 rods for the extra length.  The rods allow me to use a slant six piston which has the appropriate compression height.  I acquired the rods which were all supposed to be resized, however the rod bolt threads were awful.  No big deal, I want better rod bolts in them anyway. so just match up a set of 3/8-24 rod bolts with the right length and there we are.

  Well, here comes the issue:  the 218 rod bolts seem to be @.015" smaller diameter than they should be.  I have 3/8-24 rod bolts from the pontiac and the nut will go on the 218 rod bolt, although it is really, really, sloppy.   However, the 218 rod nut will not go on the 3/8-24 rod bolt from the pontiac.  If I measure the thread outside diameter on the pontiac rod bolt it is @.380.  When I measure the 218 bolt outside thread diameter, it is @.363 (this is in a good thread area).  The pontiac rod bolt shank is @.382 and the dodge rod bolt shank is @.368.  I have 9 Dodge rods and they all measure the same.  The rod part number is 617925.  The pontiac 3/8-24 rod bolts will not go into the dodge rods.  If it matters, these rods are the lock washer type rod bolts if it matters.

  Does any of this make sense?  Is there something odd about the rod bolts used in the Dodge 218?  I would typically have the rod bolt holes reamed for a 3/8 rod bolt but the holes are so close to the bearing, I think it could put pressure on the side of the bearing.  dan...

[oldodge41]

I know it doesn't answer your actual question, but Vintage Power Wagons sells new rod bolts and nuts.

[Sniper]

Might talk to the freewheeling tony smith on facebook, he's real familiar with these rods and such.  In fact I believe he has a set of ARP rod bolts for these rods.

 

Here is what he says about the rod bolts

 

Quote

Rod Bolts
            Rod bolts are the most highly stressed fastener in an engine. There is no good way to determine when one of these old bolts is going to fail so at the very least they should be replaced with NOS bolts. I prefer to upgrade the rod bolts to ARP which are much stronger and therefore generate a greater clamp load. Some of these engines used a special lockwasher that fits in a recess in the cap. I'm no fan of lockwashers inside engines but unless you have the caps modified (spotfaced to remove the recess) you must use the factory lockwashers. 265 rods used specialized rod bolts and I have not been able to find a suitable replacement from ARP.

 

Edited April 17 by Sniper

[dhughens]

  Thank you both for responding and your suggestions.  I spoke with vintagepowerwagons earlier and they referred me to Tony Smith at moparmontana.com.  I've sent moparmontana this same question and am waiting for a response if possible.  I could put stock rod bolts back in but as was said, it doesn't explain what it going on.  It's almost as if the bolt thread is between 11/32 and 3/8 which is not possible.  We certainly wouldn't have used Whitworth thread and there was no such thing as metric here at that time.

  I've spent a lot of time online searching this issue and see absolutely no other mention of an odd rod bolt fit.  I've downloaded Chrysler and Plymouth service manuals of the 40's and 50's and nothing there.  I've downloaded Dodge flathead builds and nothing is mentioned, although I've seen torque specs which refer to the rod bolts as 3/8-24.
  If the rod bolt holes were larger diameter or the rods came from a single source, something could be inferred but neither is the case.  Somewhere in all the performance flathead builds, someone must have installed ARP, Pioneer, or other aftermarket rod bolts and encountered this.  I mean, Spitfire flatheads ran hard in the past circle track days.  I simply cannot find a single mention of my rod bolt diameter issue.  It is not possible that I am the only guy in the entire Mopar world in some 75+ years of flathead sixes who has encountered this situation.  Anybody else curious or have some input?  dan

[Sniper]

Maybe some meathead put in different bolts in those years?  Maybe metric?  Do you have a thread pitch gauge?

[dhughens]

That's a reasonable thought except the existing 218 bolts are snug in the rod and cap holes.  I absolutely cannot put a standard 3/8-24 rod bolt into the 218 rod or cap.  ( I could problably smash them in and destroy the threads in the process. )  The 218 hole diameter is too small for a 3/8-24 bolt.  Attached is a pic of the two bolts.  Best harbor freight caliper dimension of hole in Pontiac rod cap with 3/8 bolt is .384.  Best harbor freight caliper dimension of hole in Dodge 218 rod cap is .370.  Doesn't make any sense.

[oldodge41]

Keep us posted when you resolve this please. I am curious.

[Matt Wilson]

Here's a possibility.  I wonder if the Dodge rod bolts have threads that are cut, while the Pontiac rod bolts have rolled threads.  Rolled threads are formed by dies that displace (smash) the material into the shape of threads, without removing any material.  This results in threads that have larger outside diameter than the adjacent shank section of the bolt.  On the other hand, cut threads are just that - they are made by cutting material away to achieve the thread shape, so the threaded section diameter will not be any larger than the diameter of the adjacent shank.  It's kind of hard to tell from your photo, but it looks like that might be the situation.  If so, then accordingly, the Dodge rod might be made with smaller bolt holes to snugly fit its cut-thread bolts, but the Pontiac bolts, with their rolled threads are too large to fit.

 

I have a couple of Dodge 230 flathead rods at home, along with their bolts, and I can tell you that the threads in those bolts have the appearance of being cut, which surprised me, since I have seen  other Dodge flatheads that have the appearance of having rolled threads.  I have no reason to think that those are not the original bolts, and perhaps they were early rods that used cut-thread bolts.  I could take some measurements and photos of those rods and bolts some time in the next few days and we can see if they match the dimensions of your bolts.  We can also see how thin my other rods are (the ones that used rolled-thread bolts) in the wall between the bolt holes and the bearing saddles.  I remember them being pretty thin.  We might find that you have room to ream out the bolt holes in your rods to accommodate the slightly larger Pontiac bolts, or some aftermarket bolts, such as those from ARP.

 

From the strength and durability (fatigue) standpoint, the bolts containing rolled threads are very much preferred.  I believe virtually every manufacturer uses those types of rod bolts nowadays and have for many, many decades.  I remember seeing early rod bolts from other engine makes that used cut-thread bolts, but I think most everyone moved away from that type of design long ago.

Edited April 18 by Matt Wilson

[dhughens]

That is a very reasonable answer.  I have some smallblock dodge ARP rod bolts and just like the pontiac bolts, they also will not go into the 218 holes.  I would appreciate your efforts in measuring your flathead rod bolts to compare numbers when you get the time.  Thank you and anyone else who has early flathead dodge rods available to measure the rod bolts.  This seems like a plausible answer.

 

WELL AS A MATTER OF FACT:  You are the man of the hour.  I just went out to the garage with my nut/bolt gauge and also a 3/8-24 standard bolt with nut and amazingly so, they fit the Dodge rod hole, rod bolt, and nut exactly.  Additionally, the pontiac and ARP bolt will not thread onto the nut/bolt gauge or the 3/8-24 bolt and nut.  We have a winner!!  Sorry, no chicken dinner.  But many thanks for the solution.  I never would have thought in that direction.  Now it's a matter of if the rod and cap will take a reaming without interfering with the bearing.  Thanks again for all the responses and especially the solution.  It does seem odd that in all the builds online, no one mentions this.  Thanks again all.  dan...

[Matt Wilson]

I dug out some rods and bolts and took some measurements this evening.  The first photo shows a few rod bolts lined up side-by-side, with some dimensions and commentary for each one.  You can see that the threaded section outer diameters don't differ greatly from bolt to bolt, and not as much as the bolts in your photo.  The threaded section of the original 230 rod bolt (3rd from left) has an OD that is not consistent.  It seems to be largest at the end of the bolt, then gets a little smaller heading toward the shank, and then gets larger again as we continue approaching the shank.  I take that to mean that the bolt has been "necked down" due to yielding of the bolt in that region.  Best not to use a bolt in that condition.  You can see that my assessment of that bolt is that it has cut threads, whereas the other three appear to have rolled threads, since the threaded sections are larger in diameter than the adjacent shank section in each bolt.

 

The next few photos show a couple of 230 rods and caps, focusing on the thickness of the wall between the bolt hole and the bearing saddle.  One of these rods is an old rusty one that is not usable and has been hanging in my garage as wall art.  The original-style bolt in my first photo was taken from this rod.  I measured the thickness of that wall and then sanded away the rust from the bearing saddle (bolt hole wasn't rusty) and the measurement didn't change (reads about 0.015").  As you can see in the end view of that same rod, the wall at the opposite hole is a little thicker (I think it was about 0.030").  I also measured the cap from another 230 rod that has been preserved in a plastic bag with some oil (see photo).  It shows about 0.006" wall thickness, which seems extremely thin, but this cap and its rod were running just fine in an engine I rebuilt over 20 years ago.  I disassembled that engine for other reasons about 9 - 10 years ago.  This cap and its rod were in service with the ARP bolt shown in the first photo.  As long as there is no burr or other raised metal to interfere with the fit of the bearing within the rod, I suppose there won't be any issue with such a thin wall.  I also measured the wall thickness of a 251 rod (not pictured) and it was about 0.012" at one hole (opposite hole's wall was thicker).  Again, this is a rod that had been run that way, and I believe it was factory-original.

 

Hopefully this is of some help.  Let me know if you want more photos or measurements.

[dhughens]

When I take the rods in for resizing with "rolled thread" bolts, I am going to ask if the reaming can have an outward bias.  Don't know if this is possible.  MATT, I hope you saw that you nailed the problem.  Isn't it odd that no one has ever mentioned this situation?  dan...

[Matt Wilson]

11 hours ago, dhughens said:

When I take the rods in for resizing with "rolled thread" bolts, I am going to ask if the reaming can have an outward bias.  Don't know if this is possible.  MATT, I hope you saw that you nailed the problem.  Isn't it odd that no one has ever mentioned this situation?  dan...

Yes, I saw your post from the other day and I'm glad to be of help.  I think nobody has talked about this because not very many rod bolts have cut threads.  I think those went out in the early days, and it's probably one of those things that most people don't pay much attention to or know about.

 

That's a good idea to ask about offset reaming.  Good luck with getting your rods and bolts to play well together!

Edited April 23 by Matt Wilson

[dhughens]

I speculate that pre ww2 bolts were more likely to be cut but with the advancements in the military equipment, i.e. planes, etc, stronger threaded fasteners were needed.  Just a thought.  dan...