Technical Specifications for Wrist Pins?

[CJFusco]

Good morning, all; long time no see!

I'm in the beginning phases of an engine rebuild of a spare 8v NA engine that will go into my current 85.5 944 Track/AutoX car build (the engine in the car has 320K miles and is in need of a rebuild). I need to replace at least one of the piston/rod wrist pins (I'll likely just replace the whole set), but I'm running into conflicting info regarding the exact specifications.

It looks like the parts supplied by Wossner measure 24 × 63.5 × 16mm (4mm Wall) and weigh in at 121 grams (although it looks like 124 grams is also an option?). These are available straight from Wossner, although Vivid Racing also stocks them and lists them as fitting an 88-89 944 -- does this mean they are specific for the 2.7? Does it make a difference?

Lindsey also sells an "upgrade" part which the site claims to be lighter than the stock wrist pin... at 135 grams. Huh?

If all these options would work, I'd obviously prefer to go with the less expensive, lighter Wossner pins. But let me know whatever input you might have.

For context, the car's drivetrain will be mostly stock, although I am planning on advancing timing a smidge (4 degrees or so) to move the torque curve down for AutoX, so super-high revving won't be much of a concern.

Links to the parts:
Lindsey: https://www.lindseyracing.com/LR/Porsche/LRA-PISTON-PIN-951.html
Wossner: https://wossnerpistons.com/products/wp022-sp?_pos=2&_psq=24x63.5x16-121-grams&_ss=e&_v=1.0
Vivid: https://www.vividracing.com/wossner-wrist-pin-24x635x16-wall-121-grams-p-156772905.html

Thanks in advance!

[Monkey Wrench]

in the volvos Ive worked on I'd ordrer new pistons and wrist pins, then replace the wrist pin bushings. use the crank and rods over.
If I recall I had them installed by a shop and he used a little furnace and also resized and bored the connecting rod big ends, they go egg shaped. this makes them round again to fit the crank and not be egg shaped.

after that a machine shop can balance things, there is a bit of a science but essentially all the same weight then the ends are balanced end to end. next they can involve the crank presure plate harmonic balancer etc. - all the rotating parts separately and also as a unit I believe. then you can go on to cc'ing the ports, polishing matching flanges up etc.


one method of measure is to pick up the piston and it should fit well enough that there is resistance but maybe not so much that it will flop about if you grab the rod and let the piston free. what I was told, it shouldn't just flop down. the wrist pin bushing is reamed to fit the new wrist pins. once the piston can slowly drop under its own weight, its about correct.

how to describe the fit, well it is not a press fit it is a push fit how tight of a push fit that's geeting down to very small amounts, you dotn want things so tight there is no room for oil between. a bit to far, its a sliding fit. that may not be optimal or problematic. the amount of metal removed from the bushing between a push fit and a sliding fit is not much material.

I feel that the actual "fit" means more than comparisons of the ID and OD..

that's because at that stage of fit there is a lot at play other thhan size, its how round things are and how concentric. the parts that are close enouhg to cause friction need to be removed but only those not everywhere..
hand "fitting" of the parts to one another might be more accurate than you can measure with callipers and such.


now when I did that on a volvo rebuild I was told I was going a little too far and wrist pins didn't need to be changed. I had an experienced guy say look we never change those !

I think it comes down to the fit, so while you can get the ID and OD and subtract,it is a difficult thing to measure, which is why I'd ream slowly,only iuntil they can go in, and then fit by hand after..

a tad too far and it will be all loose. You might still not hear it knock or anything but I guess there are some differences between "it will run" and all is "tight and it is rebalanced".

with a crank bearing you can plastiguage but not the case with a wrist pin bushing because it can't separate in half.

with my rocker shaft, what I did was had them mke install and ream the bushings and leave them all all a tad tight.
I then used brasso and worked them on by hand until they just fit nicely. no more binding. they move smooth with light oil.

I machined the shaft slightly tapered and so I could start at the small end and work across to the larger end while fitting them .

perhaps when racing, tight tolerances may decrease output until it is broken in and a bit of a loose fit won't make it loose races so long as its' not loose enough to cause low oil pressure or a failure.
that may have more to do with piston clearances , I do not see properly fitted wrist pin bushings causing noticible grief as long as they don't seize.

if you want to increase or run up at the redline, balance is important. If you do a rebuild and do not balance anything but have acceptable tolerances it will still run. I doubt youd hear a knock until you are past about .004" too loose, or maybe even more befoire it would be audible.

from my experiences I would not skip the balancing process , I feel it makes a huge difference especially if you are pushing the motor to its limits it will sound happier. youll feel happier with the sound it makes. the reduction in vibration.

When I did have things profesionally balanced the cost was a pleasant surprise for me. I don't habve a lot of experiences with it but my take was it was money well spent and not as much money as I first thought.

one thing that I was surpised of was that although I had bought brand new quality pistons the banancer still removed material.

Ive heard that in factories they ahve a weight difference and balancing may include choosing parts of equal bbalance, you might go a little firther in involvig a shop who can do it more accurately..


the cost of adjutable reamers, or shop time might be close, so you may not opt to buy the tooling. If you do lots, then sure, buy more specialized tooling.

I assume you aren't changing the fit of the wrist pins to the pistons themselves. i would think those would be fit in the factory but maybe you can , I never have.

one thing I' d be aware of is any burrs, if a piece has a slight burr and you press it in without removing the burr it can act like a reamer and remove material. best to check any edges for any signs of a burr. maybe polish the corners off with a fine bit of paper or a fine file, so they can't catch and act like a reamer during assembly.

i believe there are others in the forum who have more experiences forst and with the porsch engines and they may be ablle ot offer specs and maybe more experienced discussion, thats just stuff Ive accumulated not really by the book, and mostly not Porsche engine specific.

you need room for oil no binding, any looseness beyond that is essentially "wear" a bit of wear wont stop it or make it fly apart but tighter is better so long as it is.t in danger of binding up. It will probably fit itself in somewhat.

what Id do wihtout looking things up would ne to attain a good fit in the wrist pin bushigns and then maybe use a compound to hand fit the last little bit. there maybe different compounds, Ive used brasso, I have some sulpher stuff that is made for for non ferrous materials, it is a fine yellow powder probably a really old can.

if you use somethign like a grinding comound to fit bronze bushigns what can happen is the compund gets embedded by the bronze and now you have wear due to it not all being removed. so Id wash parts well beofre assembly. using a type of grit even though it is fine maybe a bad thing. thats why a reamer is preferable. it won't leave embedded grit behind.

I remember along ago I had ot fit some shafts and bushigns and they arrived tight. it wasnt; an auto part but a critical shaft of a printing mpress, very precision parts.

i went and used a brake cylinder home, I was later corrected that this worked, but I should not do that again, it was bad practice. the fact that the bronze will accept the grit from the cylinder hone, and embed it within itself was the point and lesson learned there. the machine worked fine and the technical point was instilled..

my take is that if you take those parts ot an engine shop with the experience to do precision balancing and fitting it might be worth the cost to have them balance things and fit the bushihngs and maybe advise a bit more. Id at least call around and see if you can get an expert rebuilder to weigh in . they domt need to block to balance the reciprocating parts, resize rods or check that wrist pin fit.

[CJFusco]

Thank you for the lengthy reply. I guess a little more context is needed.

The reason why I feel the need to replace one (or all) of my wrist pins is because when I acquired the spare engine, the wrist pin was left inside a connecting rod which was exposed to conditions that allowed rust to bond the pin & bushing; the wrist pin is now scuffed/no longer smooth on a decent-sized section. Hence why I was looking into replacing at least one, and curious about the technical specs and the apparent disparity between sources.

It looks like my best solution might just be to pull a full set of used ones from the EXTRA spare 944 engine I have in my garage and clean them up.

[Monkey Wrench]

its important that the running surface is smooth, polishing it up would likely make it fit too loose.
Maybe there is some advantage to keeping your spare engine intact, so you could perhaps weigh the cost of other pins and pistons and bushings? into the decision. No doubt it would run even with some looseness but you want it to last well. Obviously going that far into the engine has an expense in time and money anyway.

considering the time and costs of going that far, Id be tempted to want to make it right. balancing of the parts might be considered an extra to consider, depending on the expected outcome. cash flow is important too, at least for most of us.
rust will cause pitting removing the pitting through polishing will decrease the ID, You can change bushings but not the fit of the piston to the pins. new parts are an option or used parts.

maybe ask the others about parts, you might desire to keep the spare engine intact, so Id compare the cost of parts ( new or used pistons and pins? ) to what it may be worth to you to leave it alone and maybe use it for anpother project. the rods can probably be re-bushed and precision fitteed to the pins, you might consilt an engine shop for help with that.

most of my experineces have been with cast iron blocks where you could bore 20, 30 40, 50 thou factory oversize and get a new piston fit but I believ with this engine since it has a special coating this would involve having the coating reapplied. maybe there is some advantage to not swapping pistions from one block to the other?

probably the others here could advise better on replacing rings and fitting them and breaking the new rings into a block with previously used and worn bores. maybe youd find the pins form the other engine fit aceptably well ? and keep the pistons with the bores they have worn into ?

with cast iron engines the bores develop a ridge where the piston reverses motion, if the ridge is not removed and new rings are fitted, although the interference area is slight it can break the top compression ring. a special tool called a ridge reamer can remove the ridge and the bores can be homned and new rings fitted. if the pistons still fit well enough that is somethign that can bbe gotten away with. with a loose bore there is some piston slap and the pistons and bore can wear from that, onthe sides, from the action of them reversing direction.

wiht this engine , and its special coating, I dont know if this poses the same danger wiht the bores the 944 has. that's not a quiestion I could answer from experience but others have more experience and could likely refect upon all that.

I think it is possible to have the block recoated or to get away with reusing the bores and not doing the recoating .. Im unsure of the cost of having it recoarted. I assume if you did all that you could go to an oversize piston but i assume to get a proper and new fit, it would come only with a significant cost.

in older cast iron block engines it was common to increase the bore to increase power output but the lomit is often that you are then thinning the waterjacket and if you go too far you might blow through. I rebored my volvo from an 1800 to a 2 litre so that involved a lot of cast iron remopval.

I suspect these limits on increasing bore size and displacement are much more restricted with this sort of engine block , even with recoating.

I often deal with q hydraulics shop, they have thre ability ot increase the size through application and grinding of applied chrome and they acna do extremely fine tolerances. on some shuttle valves there are no seals and the bores are fitted to tolerances as fine as .0001"

as things like tractors have exposed chrome pistons, the pistons can get scratched and that can wreck the seals causing leaks.. rechroming and grinding them to size is a regular activity for them.

I think Id try taking those pins wiht the rod and piston to a hydraulics shop and just ask, that may be a practical way to fix them , maybe you could leave the spare engine alone then.

most major centers have hydraulic rebuilders, they have the ability to do this work. Id just walk in and ask if they might be able to help. They might be helful or they may say its not what they do.

I bet a hydraulics shop could clean up the pins, rechrome them and grind them down to size and fit them accurately to the pistons and cranks that you have.

rechroming is an environmentally sensitive process , hydraulic rebuilders have these abilities because it is quite necessary for industry.

I have a small lathe at home, I can machien parts , some types of steel and I assume chrome doesn't work well with regular cutting tools becuse its too hard to be "machinable"

instead Id use a toolpost grinder.. This is a grindstone that follows and is mounted in place of the cutter, the grindstone and polishing wheels can remove material that is too hard for normal cutting operations. I would assume a hydraulics shop with have a similar type of setup. the chrome woudl increase the ID of the pin then the machiens would reduce it to size.

maybe new pins can be made, I wouln't be confident enough to attempt it mysef. perhaps this is also possible.