4.8L / 96MM Piston problems

[debaron]

Boosted LS1 said:

This seems to be confusing. 35 thou is massive for piston to bore clearance, assuming you mean .035". Some pistons can run as tight as .0005" whilst forgings may be happier at .005". I also think that once the piston's expanded, that's it. It won't keep on expanding as the skirts are shedding heat.

Yep - it's 3.5 thou = 0.0035 clearance as specified by manufacturer

[Boosted LS1]

Does the manufacturer say where to measure the piston?

[RobXjcoupe]

Boosted LS1 said:

Does the manufacturer say where to measure the piston?

I would assume if you buy pistons you would know where to measure from so the manufacturer would just need to quote a clearance but it's not a tolerance. The tolerance would be an upper and lower figure applied to the piston actual size.
Blueprint engines are built to dimension without a tolerance i.e. Spot on and very expensive. Not the usual for a road going engine builder.
It's hard to determine what's needed with just a seemingly random piston size and a 35thou clearance given for an n/a useage engine or 45thou forced induction engine.
I'm sure Dom at tvr power will point the owner in the right direction

[debaron]

Boosted LS1 said:

Does the manufacturer say where to measure the piston?

Yep - at the dial point (.500” from bottom of skirt up and at 70F) piston should measure 3.7775” -3.7780”.
Recommended clearance for this bore and boosted - .0035”
Final bore size: 3.7775” + .0035” = 3.7810”

[Boosted LS1]

^ That sounds reasonable enough. I've had specs as large has .007" which didn't rattle and on subsequent builds I've dropped that to .004" without any issues. Short skirts aren't always an issue but ' piston rock' sometimes is. Some manufactures quote large so that it becomes nigh on impossible to seize an engine even if it may sound like a bag of spanners.

Also, the rings offer a lot of support.

Edited by Boosted LS1 on Monday 1st May 08:52