Hi folks,

I am in the midst of a rebuild on my 2wd YB. I'm located across the pond so I'm somewhat on my own for sourcing parts and doing the build. I have good experience with engine rebuilds so I'm not concerned there. Anyway, on to my issue at hand.

My engine was previously, rebuilt in Canada before I took possession of the car and it was not done to acceptable standards. Of most significant note would be the use of NA forged pistons and a .120" head gasket! I'm not sure what the resulting CR must have been but I am reasonably assured that it would not have liked my GT35R turbo. The cylinder bores were oversized .030". I had the choice of using JE pistons but the price was not that far off from CR 8:1 1mm overbore pistons from Burton. I chose these and waited the 3+ months for them to finally arrive. Much to my dismay, there is NO literature included. I have NO clue what the recommended piston to cylinder bore clearance should be.

Long story short, I know that the shop manual specifies .0015"-.0024" clearance but that is to pressure cast pistons and not full forged. From a general search, most forged piston makers say .004"-.006" clearance. It would be of great help to me if anyone here knows the OFFICIAL clearance recommendation from Cosworth. I have an email out to Burton and Cosworth but they are slow to reply.

Last question...Any reason NOT to size the small end bush to piston pin clearance to less than .0005"? The bush clearance on my engine at disassembly was .0012"

Thanks, Rich

 

id contact burton, there really should have been info on bore clearances

 

From a Cosworth spec sheet (doesn't list YB pistons) it looks like 0.003" to 0.005" is typical.
For high boost build I would use the higher clearance spec of 0.0045"

The Mitsubishi specs are with a cast Iron block - I would assume those specs would hold for a YB build as well.

Wristpin clearance of 0.0005" looks good (clearance spec in following data sheet)


http://tomitaka.com/Products/Engine/...ts%20v1.0a.pdf

 

Your best off asking a specialist who builds yb's day in and day out to a high standard. There's a few good ones I'd recommend dropping mark sheed of m.a.developments an email

 

Thanks for that sheet, it's at least something. I was thinking .004"-.005" for my build would be safe. I don't necessarily mind a little piston slap when cold but I want to minimize wear if I can help it. I will try to bug both Burton and Cosworth until I can get an answer. I'm still baffled why something so important would not be included with my pistons.

 

Different manufacturers use different alloys that require different clearences for forgerd pistons.

Only burton or the piston manufacturer can give you the right answer.