964 3.8 RSR: Small End Rod Side Clearance to Piston (how much is not enough?)

[CBRacerX]

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.



it reads 24.77mm

[Bill Verburg]

Are you sure that you don't have 993RS pistons? They are not useable w/ 964 rods because of the rod width reduction on 993s

[CBRacerX]

I was sold these as 964RSR parts. The piston is marked as shown here:

[CBRacerX]

From a Rennlist conversation http://forums.rennlist.com/rennforums/showthread.php?t=288067 it appears that new rods are required (ouch). One good suggestion was to measure the piston pin height to be sure that there is no requirement for a longer rod. Does anyone have a procedure I could follow to collect this measurement?

[CBRacerX]

The silence is deafening these days

My other question is the minimum required gap from the small end of the rod to the piston support - anyone have a good value for this?

[herman maire]

I would break the silence but I am of no use....thats why I stay in the shadows. Bummer on the rods... what are you gonna do for custom rods?

[CBRacerX]

Right now R&R Rods from LN Engineering have my interest. Any length, any width for small end.

[cnavarro]

The only problem would be lead time - we're putting a hold on all rod orders until after R&R moves to their new, expanded facilities. One good this is that they are tooled up and are running a large order of rods until the end of the week for an oem they supply.

Chris, to answer your question on clearance between the pin end and the piston can be run as tight as .050-.060" per R&R's recommendation, but larger won't hurt, as evidenced by the huge clearance left by the factory (.280" or .140" per side per your measurements).

[herman maire]

FWIW...I dont know what material LN Engineering use but in anderson's book he sais to go with titanium rods.... Never aluminum

[CBRacerX]

Thanks Charles - is this .050" total clearance? ie. allowing .025" per side?

And with respect to availability - does this mean get orders in right away or wait?

[CBRacerX]

I've done AL once (CMW). NEVER again!

The LN Engineering Rods are made of very nice stuff

[cnavarro]

It's .050-.060 min per side - I should have made that more clear. :-)

[cnavarro]

Well it all depends on your schedule - if they get in before they close shop to move across town, it might be six weeks till you have them in your hands vs quite a bit longer, say mid sept or even later. They don't have specifics as the building is still going up, but is supposed to be up by the middle of August. Since there are a few more steps they go through than most guys with the cryoing, heat treat, and tempering, it takes a bit longer to get the rods done.

[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.

[Wayne 962]

Tough stuff - in general, the RSR parts are designed to work with other RSR parts. I don't have too much experience with the later model RSR engines, but I do know that you have to mix and match properly, otherwise, one part of your engine will be stronger than the other. The stock 3.6 rods are a weak point in the engine...

-Wayne

[CBRacerX]

Charles was kind enough to loan a very nice billet 993 rod for me to check the deck height with, and Mike Bruns @ JBRacing.com is sending me a bearing to use in the test.

Thanks to all!

[Cupcar]

Carillo custom rods a solution?

http://www.carrilloind.com/

[Cupcar]

Looking at the Carillo catalog I note that their RSR rods with the wide main bearing shells have the same .768" wide wrist pin bushing as a 993 rod.

This is evidence to me the RSR pistons require the 993 width rods.

[CBRacerX]

R&R Rods from LN Engineering have been ordered with a 993 width on the small end.