cbracerx

964 3.8 RSR: Small End Rod Side Clearance to Piston (how much is not enough?)
What is the spec for clearance of the rod small end to the inside of the piston wed that supports the wrist pin? I have what could be an expensive issue with my 964 3.8 RSR pistons and my 964 stock rods. The rods have a small end width of 22.58mm without consideration for the bushing. The RSR pistons have a gap of 23.10mm. That leaves a whopping 0.52mm of free space to play with, and that has to handle both sides of the piston pin. For reference, the stock 3.6 964 piston has a gap of 29.82mm, leaving 7.24mm of open space.

With the bushing installed, my rods have a small end width of 24.77mm, giving me -1.67mm of clearance. That, of course, will not work.

So even if I machine the bushings flat with the surface of the rod small end, I will still have only 0.52mm of free space. I looked in the 964 spec book and could not find a measurement for this, likely because the stock parts will never cause any concern here.

Geoffrey

You need an RSR rod to run with RSR pistons.

Larry Herman

Geoffrey, maybe Chris can have the inside of the piston machined out to gain a little clearance?

Premier Motorsp

Those pistons look really hogged out already. I haven't seen any like that before.

In any case, a 964 rod isn't really suitable for an engine that would use RSR pistons. Get Carrillos. Their 964 rod has the correct small end width. If you order 930 rods everything is the same except the small end width is larger.

Chris Cervelli
Premier Motorsports

Geoffrey

Chris Cervelli, I believe those pistons are a late set of Sprint Engine pistons with higher compression than the standard RSR pistons.

Chris Brown, You'll also want to verify the pin height to ensure that the pistons will work with a 127mm rod and they (PMNA/Mahle/whoever) didn't change it to require a longer rod.

cbracerx

OK guys, appreciate the feedback. I'm fine (although pained in the pocketbook) with getting other rods.
Is there an easy reference method for the pin height I could use? Is this the measured distance from the center of the pin location to the top of the piston or ???

I've enclosed a few more pics to help with the mystery.

Thanks!

cbracerx

This somehow was not added in the last post ?

cbracerx

So given that a rod change is in order, what/where are my options? Chris C suggests Carillo's 964 rod - is there a good palce to source this? Geoffrey suggests RSR Rod (I assume factory) that probably has a scarier price and availability that the Carillo, but I have no real experience to back that up. Is Pauter even an option? How about LN Engineering, they are now offering a rod from R&R.

Thanks again for the help guys!

Geoffrey

I should have said an RSR sized rod. I'd agree with Chris and suggest getting a set of Carrillo rods over Pauter rods.

cbracerx

Thanks Geoffrey - I'll look to sourcing the Carillo 964 Rod. Any hints from the collective on measuring the pin height? I have stock 964 pistons here as a reference, but this would be first time I have made such a meaurement.

cbracerx

Measuring a stock piston and the RSR piston using a stock wrist pin showed:

36.78mm Stock from upper inside of pin to top of piston dome
36.68mm RSR from upper inside of pin to top of piston dome

So I think a stock rod length is in order.

NineMeister

I would agree that Carillos would be the way to go, you can order them to suit your little end width (same as a stock 993) but with the wider 964 big end journal. As mentioned all standard rods are 127.0mm centre distance, but if the piston is slightly different to standard it will make your life a lot easier to have the connecting rod length adjusted to suit. Make sure the pin diameter is stock as well, some of the later Mahle pistons had 22mm pins, not the standard 23mm.

The critical measurement is the squish height from the centreline of the pin to the flat squish area around the edge of the top, not to the top of the dome. You should aim to set the squish gap between the piston and head to around 0.8-1.0mm (0.032" to 0.040"), the easiest way to measure this would be to borrow a stock 993 rod and dummy build your engine with the new piston, then measure down from the top of the barrel to the piston along the pin axis. If the squish is in the ballpark you can always deck the bottom of the barrel to get it perfect, but if it is more than 0.25mm out you currently have the option to have the rod made longer or shorter to suit.

cbracerx

Colin - thanks for the detailed response. The pin is 23mm, so I am ok there at least. Now the challenge is the 993 stock rod - but I think I can get a loaner from a US rod vendor.

Cheers,