Custom pistons

[tysouthwick]

Well, does any one know where to get any made? I read the horror stories of an unbalanced engine, Scary

[aero3113]

I think you only have to worry about balancing an engine if it is a high performance race engine.

[randroid]

tysouthwick,

The more closely the engine is balanced the longer it will last, all else being equal, but it isn't a dark and mysterious process and certainly doesn't require any custom made parts. I've balanced a number of engines on my own and it's fairly cut-and-dried and you can make scales yourself that will get you a lot closer than the factory bothers to do. Racing engine are balanced to small fractions of a gram but I doubt if any stock engines are much closer than 10 grams, so pick a weight tolerance you want (the closer you get will cost you more but that's because it takes more time).

Let's say you want 5 grams tolerance, which is only slightly heavier than a 25 cent piece. Balance your pistons to within 5 grams (there are places on pistons where it's safe to whittle them down) then do the same with the rods. Pair the heaviest piston with the lightest rod and keep matching them like that and they should all be at or below 5 grams of each other.

For a NASCAR engine you'd also want to balance the crank, cam, flywheel and pressure plate, etc, which is out of our realm of ability, but at least now you know why it's important to mark the flywheel and pressure plate when you remove them; they've already been balanced to each other at least some.

If you're nervous about doing it yourself the first, don't worry because that's a sign of good judgment on your part, but a machine shop should be able to give a good estimate and it doesn't require any special parts.

-Randy

[BobT-47P15]

I think balancing will make these old Mopar engines run smoother and

maybe last longer. Don't know that it'll make them run any faster.

[Big_John]

Ross Racing Pistons and Diamond Pistons will build custom pistons.

While piston weight variation will be minimal in a set of custom made (read expensive) it will still be checked and adjusted as part of the engine balancing. You will need to balance your rotating assembly if you significantly change piston or rod weight from stock.

That said, if you are going to the time and expense of having custom pistons built, it doesn't make sense not to spend a little more money (even if the pistons are close to stock piston weight) and have the assembly balanced.

[martybose]

I had my pistons made by Venolia. I just sent them a stock piston, a set of the rings I wanted to use, the bore that the rings were intended for, and the wrist pin size I wanted, and they made a nice set of forged pistons for me.

Marty

[Don Coatney]

Well, I read the horror stories of an unbalanced engine, Scary

On Mopar flathead engines that only turn 4000 or so RPM's the balance is not as critical as an engine that spins 8-9 thousand RPM's. Do you need a special sized piston that is not available? I would think it would less expensive to bore your block to a standard size and use off the shelf pistons than to have them custom made.

[PatS....]

I have 2 sets of 6 "new" NOS 30 over pistons and rings if anyone's interested. Made in 1958.

[TodFitch]

tysouthwick,

The more closely the engine is balanced the longer it will last, all else being equal, but it isn't a dark and mysterious process and certainly doesn't require any custom made parts. I've balanced a number of engines on my own and it's fairly cut-and-dried and you can make scales yourself that will get you a lot closer than the factory bothers to do. Racing engine are balanced to small fractions of a gram but I doubt if any stock engines are much closer than 10 grams, so pick a weight tolerance you want (the closer you get will cost you more but that's because it takes more time).

Let's say you want 5 grams tolerance, which is only slightly heavier than a 25 cent piece. Balance your pistons to within 5 grams (there are places on pistons where it's safe to whittle them down) then do the same with the rods. Pair the heaviest piston with the lightest rod and keep matching them like that and they should all be at or below 5 grams of each other.

For a NASCAR engine you'd also want to balance the crank, cam, flywheel and pressure plate, etc, which is out of our realm of ability, but at least now you know why it's important to mark the flywheel and pressure plate when you remove them; they've already been balanced to each other at least some.

If you're nervous about doing it yourself the first, don't worry because that's a sign of good judgment on your part, but a machine shop should be able to give a good estimate and it doesn't require any special parts.

-Randy

Pretty good overall, except "I doubt if any stock engines are much closer than 10 grams"...

Factory specification for pistons in 1933 was 1/4 oz (7 grams). Amazingly, I don't see a similar number in the 1946-54 service manual but I can't believe they got worse on factory specifications in those 15 or so years.

The set of pistons I received from Egge back in 1976 or so had quite a bit larger variation in weight than the 7 gram specification which led to my one experience in trying to balance engine parts.

[randroid]

Tod,

I was guessing on the weight as a factor of the lack of care given to building and assembling modern cars. For example, manufacturers used to 'season' their engine blocks by sitting them outside for a year or two to allow the iron to settle into its final shape, and then they were brought back inside to be milled and bored, but that foolishness was stopped for two reasons: First, they ran out of room to store the blocks while they were curing. Right, they were selling so many cars they could no longer find anywhere in Detroit to stash even a few? Second, as roads and driving conditions improved the engines lasted longer so by skipping the expense of properly curing the blocks they were able to get the same mileage before the engine went south. You ever see a V8 engine get align-bored? Looks like a porcupine, with one side if a cylinder getting hogged-out while the other side is barely touched. I think allowing them 10 grams in balance may have been overly generous.

When I had Pigiron's engine done I thought I had some money so I had it balanced to within 1\2 gram, including the camshaft and gears. Now I have a flat-head six that should be good for 500,000 miles if I take care of it but every time I lift the hood when it's running I still look around for the welding leads. That little exercise ran about $2700 and I could have bought three shortblocks from Checker for less than that so I don't particularly advise it for anyone else, and that's why modern engines suck and manufacturers don't bother with it anymore.

Don't even get me started on gas pedals sticking and killing our children.

Have a nice day!

-Randy

[Tim Keith]

The only reason I would consider custom pistons is to get

a lighter piston with 3 rings, which would probably be best

with different rods. With the 218/230 offset rods, which

I don't fully understand what is "offset" - the choices are

limited. I have thought of doing this with a 236 DeSoto

and use the 251 or 265 crankshaft with the 8" 236 rods.

That would allow for use of a lighter piston with less

compression height. I doubt if I'd notice the difference,

but I was thinking modern piston and rings would make

for less friction and maybe 125,000 miles or more. But

I won't drive that far. These old slugs can be improved

upon. The 8 inch rods must be heavy too, can't do much

about that.

[james curl]

Tim, in the cross section view of the Plymouth engine in the manual you can see the off set in the #1 rod in the picture. If you put the rods without pistons in order on a dowel rod of the correct diameter in the order that they came out of the engine you will see the off set between pairs. If you look down the bore of each cylinder at the crank throws none of them center on the cylinder thus requiring an offset rod. I think that the offset contributes to extra stress on the crank throws at high RPMs, that said I have run mine for hours at 3600 RPM with out any serious problems. I believe that the long block engines center the cylinder bore on the crank throws.

[Big_John]

The wrist pin offset goes towards the thrust side if the cylinder (the side that the piston pushes against on the power stroke). It reduces the amount of piston "slap" or noise from the piston rocking in the cylinder as the piston passes TDC.

[Tim Keith]

I've heard of custom rods made to any spec, but I think that would

be out of the question for these motors. With the old Ford 8 there

are somewhat modern rotating assembles available. I don't quite

get the fascination with the flathead Ford 8, they sure can get

spendy for not that much power increase. The OEM style Mopar

four ring pistons have got to have lots of friction, but so does the

long stroke. I would just want a dependable motor for 1,000 miles

a year, so it does not really matter. But I'd guess that someone

who went hog wild with the Mopar L-head six could (1) make lots

of power, or (2) double the longevity. For me they're okay "as is",

but I kind of wonder what could be done with one as Dick Landy

did with that Ford 8 a few years before he died:

http://www.hotrod.com/techarticles/engine/hdrp_0511_ford_flathead_engine/index.html

Edited August 3, 2010 by Tim Keith