JE Wrist Pin in Connecting Rod - Tight

[Gordo2]

Folks,

Rebuilding an engine that has ~ 30k miles since installation of new rod bushings. The bushings appear new/no noticeable wear, therefore I planned to just reuse them with the new JE pistions & pins that I bought.

Meanwhile the JE wrist pins are a pretty tight fit in the used connecting rod bushing; definitely tighter than the original pins that I removed.

Is it normal / OK to have a machinist ream the connecting rod bushings for new pins?

Thanks, Gordo

[Henry Schmidt]

Originally Posted by Gordo2

Folks,

Rebuilding an engine that has ~ 30k miles since installation of new rod bushings. The bushings appear new/no noticeable wear, therefore I planned to just reuse them with the new JE pistions & pins that I bought.

Meanwhile the JE wrist pins are a pretty tight fit in the used connecting rod bushing; definitely tighter than the original pins that I removed.

Is it normal / OK to have a machinist ream the connecting rod bushings for new pins?

Thanks, Gordo

We find that JE wrist pins tent to be slightly larger than those supplied with Mahle pistons. For this reason we always pin fit wrist pin bushing to the actual wrist being installed.

[Gordo2]

Thanks Henry,

I kind of thought it wouldn't be unusual to have to ream to fit. Meanwhile I second guessed myself as assumed the pins would be made to OEM specs with similar tolerances...

Gordo

[KTL]

Here's a comparison of some Carrera (23mm) pins of three variations.

1. Pistons are old Mahle 98mm 9.8:1 compression that were used in my 3.2SS. I measured the used pins as 0.9048" or 0.9047" (minimum 22.979 mm)

2. Recently measured a set of very low mileage pistons & pins out of an '85 3.2L Carrera. All pins were 0.9057" (23.005 mm)

3. New JE pins supplied with Carrera pistons are spec'd at 0.9050" (22.987 mm)

I'm not disagreeing with Henry in terms of JE being bigger than typical Porsche pins. I'm just pointing out the variation in sizes and the need to pin-fit your pins to the bushings in the event you're mixing parts (like JE pistons with stock rods).

JE builds the pistons to match their preferred clearance with the pin. They leave it up to the end user to decide what clearance is desired in the rod & pin combo. So, yep, gotta adjust the bushing ID to suit your needs.

[Lapkritis]

Lube can loosen the assembly as well.

[IXjamesXI]

Just another data point for this thread.. I found that the wrist pins from my 82 SC are larger than the JE's I have. Noticeably less play from the stock pin in the old bushing.. and I can fit the stock pin in my JE piston and it feels tighter in the piston.. I almost want to use the stock pin in my new JE pistons instead of the JE pin.. but maybe too tight a fit is a bad thing? Plus it weighs around 5g more than the JE pin. What is the margin for play in the pin to piston fit? I imagine too much clearance and the pin will start pounding the piston hole into an egg shape after awhile from the pin having some room to move in there? I assume too that the piston pin hole will increase in diameter when hot. Note for readers, this is conjecture by an amateur builder.

Stock pin about .8663" / 22.004 mm (this was an Alusil / KS piston set if that makes any difference.. extremely tight fit)
JE Pin about .8660" / 21.996 mm

Another interesting data point.. well to me anyway but perhaps not germane to this thread.. my stock rods weighed out all within 1 gram of each other.. I was not expecting that.

[Gordo2]

If it were me - I would provide my machinist the JE pin and have him re-bush the small end of the rod and ream to fit the JE pins.

That's assuming the JE pin fits too loosely in rod as is.

Gordo

[KTL]

Guys that do this Porsche engine thing for a living (not me) often say the pin fit for a race/track application is 0.0006 to 0.0008 and 0.0005 to 0.0006 for street.

Wrist Pin Fit in Piston Bore?

[911pcars]

Large assumption to expect new piston pins to fit an existing con rod.

It's customary to pin-fit (hone) the small end when using new pistons/pins. That's probably why JE and others supply a slightly larger than spec pin.

Sherwood

[The cap'n]

Do it right, remove the rods and have them sized and the wrist pin bushings replaced and fitted to the pins. It's cheap insurance. DO NOT use pins other than what came with your pistons!


The Cap'n

[IXjamesXI]

Definitely will do so.. just enjoying observing some of these things as I take my time with this process..

Kevin.. interesting thread you posted a link to. My stock wrist pins felt like they were welded in those pistons.. I had to fabricate a little puller for myself to extract them. MBruns explanation sounds interesting.. I will take a closer look at my stock pistons for evidence of that phenomenon.