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Porsche specialists buying, selling, servicing | 911virgin | Uxbridge

Clearance spec question - for engine guru's

H

Harv

Suzuka
Joined
18 Sep 2014
Messages
1,204
I'm trying to establish the proper clearance spec for the piston wrist pin to conrod little end bush on my 3.6 rods.

Porsche cannot tell me and would really appreciate some input.

We are coming up with .0015" - is this too much? There's certianly some rock between the pin and bush on dry fitting (no oil) but the bush measures round so assume not overly worn and engine didn't knock.


TIA

:thumb:
 
Maybe answering my own thread here but found my old porsche technical specification book for the 944/s/turbo (they haven't one for the 996 as far as I know?)

This specifies con rod bush/piston pin radial play as:

New = 0.018 / Wear limit = 0.032

Materials for the 996 I believe to be the same so assume the above specs would work

:thumb:
 
Harv, I have a few 944 pistons and I can't really feel any play whatsoever between the gudgeon pin and the little end, these were used pistons at 118k miles and may have a very little amount of oil inbetween still but I can't feel anything. I think they are the same set up as the 996 pistons as you say but when I refitted my standard 996 ones I don't remember any play.

ps, I have posted the gudgeon pin installation tool to you today.
 
Harv said:
I'm trying to establish the proper clearance spec for the piston wrist pin to conrod little end bush on my 3.6 rods.

Porsche cannot tell me and would really appreciate some input.

We are coming up with .0015" - is this too much? There's certianly some rock between the pin and bush on dry fitting (no oil) but the bush measures round so assume not overly worn and engine didn't knock.


TIA

:thumb:
Click to expand...


Harv, +0.0015" is at the upper end of normal clearance for a pin. In general, pins run between 0.0005" to 0.001", but a thou and a half is ok. Any more and it will rattle.

This info is for general engine specs, not Pork specific. I'd be comfortable at that number as long as the pin is over 22mm diameter.

Hope this helps....
 
Thanks for the feedback.

I have it on very good authority that my clearance is fine, perhaps I'm more in tune with the perception of play in these, delicate fingers....

The pin dia is 22mm.

Always intrigued how things work, I do wonder if in use this clearance closes up at all between pin and bush?

Strange that the 944 spec book gives larger figures, I just wanted to know how much was too much.

:thumb:
 
The pin clearance allows for heat transfer between the crown and the rod. Too tight and the small end will overheat (the rod will turn blue which indicates over 230 degrees C) and too much will be audible. It also acts much like an impact hammer when there is too much clearance which leads to piston failure.

Every engine has different requirements, so the manufacturer sets the limits based on empirical data from real world tests.
 
Have decided to have these little ends remanufactured to close the clearance up and will shoot for .0008

With everything else that's gone into this motor I want it to be 100% with no doubts.


:thumb:
 
Harv said:
Have decided to have these little ends remanufactured to close the clearance up and will shoot for .0008

With everything else that's gone into this motor I want it to be 100% with no doubts.


:thumb:
Click to expand...

I was thinking the same, if you were in doubt and considering what you are spending, this is small fry for piece of mind.
 
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