tommo951

What is the recommended Piston to Bore Clearance.
I am in the process of overboring and need some tollerances.
What is everybody running?

Cheers
Tom

Tom M'Guinn

For OEM pistons and bores at 100 or 100.5mm, the spec is: .008 to .032 with a wear "limit" of .080 -- all numbers mm.

Aftermarket pistons often have their own specs.

Ski

Tom if your running and OEM set, the clearance was .020 total, piston was 100.480, bore was 100.5 from the manuals, this is pretty tight; 968 104 and 104.5 were the same .020 clearance; all based on oversize boring.

The ones I'm presently getting from Mahle, with rings fitted accordingly, the clearance is .020mm but .040 total, ie .00157"; piston is 100.460 for a bore of 100.5. This is due to present expansion rate and Mahle say they've gained hp on test engines running with a bit more clearance. The Tom C pistons we have in the track car are based on .00151" of clearance, through Arias. HTH

tommo951

Bump for further opinion

333pg333

So looking at your signature you already run 3.0 L. What size are you going up to?

Ski

Tommo, you have the OEM clearance above - what pistons do you have? If aftermarket, they should have the recommended clearance on the sheet with the pistons.

nick_968

Check this old thread of mine for some info on tolerance groups and info. Shows how you do have some crossover between adjacent tolerance groups.

https://rennlist.com/forums/showthre...olerance+group

indi9xx

Tommo,
Are you running with 104mm Mahle pistons in a Alusil bore? If so, the above spec is what you want.

If not, you need to let us know every detail, such as bore material, piston skirt height, ring material, piston material (and possibly who made them) and as much detail as you can.. even down to if the skirt has a coating and if it has oil retention rings machined into the skirts... all of this can make a big difference to clearances required... go too narrow and she might pickup and destroy the pistons and bores the first time you get a differential rate of expansion inside the engine between the piston (heat soaking) and the bore (in some cases thermally insulated due to coatings) which would be bad to say the least... go too wide and you will loose piston stability, ring sealing and piston slap.

Need more info..

tommo951

Running JE Pistons of 104.5mm (Standard length not Slippers) in a Nickel Ceramic Bore. No coating to Piston Skirt Currently but considering coatings either in as Per Mahle or Composite. No Lower Oil retention rings in Skirt. All work being carried out by Perfect Bore, just want to make sure they are on the right lines. They are considering .040 total Clearance just wanted to get everybodies experiences

tommo951

Its in build at moment.....hopefully!!

Ski

Then I take it they've looked the recommended ring gap and maximum wear gap - as long as your satisfied with that issue and the .040mm falls in line or just a bit over the recommended clearance, I think you'll be fine. Good luck and post some photos when you can.

indi9xx

What made you for a Nickel ceramic bore? We prefer it over alusil now, but I thought you were quite anti Nicasil? I think you made the right choice.

Have you got your block back from Perfect bore? They can at times be a little bad with deadlines lol.

What ring material did you go for? After some discussion with our partners in motorsport we abandoned the rings that perfect bore supplied us in the past and had to have our rings made for us. Likewise with the JE pistons they supplied us... we ended up having them modified for our purpose and with improved ring lands.

That clearance is possibly a little wide compared to what we normally work to in that combination of piston and bore type, but then again if your going to have a coating applied, then it may close up a little.

There are some other issues that may snag you during your build, if you get stuck I may have some insight for you.. I cant give too much away, but if you have your block back, take a picture from the underside of the block looking up into the bores, I may be able to give you some advice from what I see, which could avoid you an engine failure in the future.

Ski

why so secret indi? I could understand electronics, turbo maps, tuning being secret but not something that might help everyone.

Tom M'Guinn

You may want to give a call to EBS racing. They sell 951 JE pistons for use in Nikasil bores, and might have some worthwhile experience to share.

indi9xx

Its not a trick or secret, just something we check, on 2.7/3.0 blocks, that we have learned to check when the same company has performed this exact same proceedure.

Nothing that will effect anyone else but Tommo, or someone else with the same kind of block being worked on by the same company.