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Piston Clearance

3K views 8 replies 9 participants last post by  Injmopar 
#1 ·
hello everyone ... as ive posted a couple times...I have a atp 50 trim turbo kit with a fuel return line,fuel pump,and injectors on a stock engine .. ive rececntly rebuilt my engine with a stock rebuild...the macine shop I took my block said they had to do a .020 overbore ... since I wanted for a stock rebuild they ordered me topline pistons which are oem spec but with a .020 bore .... when I had my block assembled another shop did the assembly... the piston have no clearance to the walls ...he told me they should atleast move a little when you try to move them but they don't at all... im wondering how many psi will I be able to boost on this engine safely as its my daily ...thanks for your help
 
#2 ·
The shop should've measured piston to wall clearances BEFORE doing any assembly (along with an assortment of other necessary clearances).

Shoddy home-built, built motors dont last long.
 
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#3 ·
Your current shop should be able to measure the piston to bore clearance with feeler gauge...if it is too tight bring your parts back to the machine shop and have them make it right.
 
#4 ·
also depends on the brand/material of the pistons. some need as little as 3 thous, some as much as 6 thou clearance, but if you had a shop assembly the motor and they didn't ask for the specs in advance and didn't measure during assembly and their way to check is to rock the piston??? what's your ring gaps? whats your bearing clearances? if they don't know, well... that sounds like a recipe for failure.....
 
#5 · (Edited)
f.020 doesn't mean that you can just bore a cylinder at exactly that and put in a piston. the final bore is set off the piston you have ordered and would have had to have been with the block at the machine shop to do it correctly.

finish bore will be different from piston material, to application, and a couple other deciding factors.

it isn't the fact that its a home built motor that is causing the issue. its a lack of knowledge
 
#6 · (Edited)
Umm. The shop is asking you for advice. Super sketchy IMO. I'd see if there is some warranty or something like that.

Some movement may be ok but it really depends on the piston and the clearance. If you were bored a true 0.020" over and the pistons spec for an 88mm (0.020") overbore then you "should" be ok, but not measuring is like having a "date" with a hooker and not covering your stuff even though she says she is clean.

Measuring the bores, pistons, crank journals, rod journals, and ring gaps should have been done at a minimum. I have heard of folks just slapping the parts together and being just fine, but when you have so much invested why in the world would you risk it. Another issue is that you have no baseline to see how things have changed over X amount of miles and you decide to "freshen" the main/rod bearings or rings.

Below is a sample of my engine build and the measurements that I read. This is by no means a perfect build as there are some variances with the cylinder walls, but are well within specifications and once you get in the 0.000x's (tenths) things are so minute that reworking the bores may actually cause more harm than good.

Single Cylinder Bore:



Crankshaft:

My readings are a little loose and I am aware of this if something were to happen. I spoke with many very competent engine builders and the bearing manufacturer and I was assured that I would be fine. This was my choice to proceed. Unfortunately you didn't have that option and I'd hate for you to have a failed rebuild.

 
#7 ·
That's a little sketchy, the engine shop I got to do my bore phoned JE with me and we all went through the engine build quick, intended use, etc and then hammered out what the piston clearances on my build should be, it's a little different for a stock rebuild, but guessing clearances is a bad move no matter what kind of build you're doing.
 
#8 ·
Machine shops and engine builders normally determine the clearances. recommendations for clearance are made by all piston manufacturers based on the material, piston growth at temp and other issues. seeing if the piston fits by rocking it is all kinds of wrong.
Another question I would have is if the pin fit clearance is off as well. If they can't fit a piston into a bore, how can they pin fit a rod and piston?
 
#9 ·
First having two shops that you don't know personally,
work on the motor is leaving yourself open to problems.

Because if there is a problem,many fingers will be pointed.

Second people confuse a remanufactured engine with a blueprinted engine.

For the long run blueprinted is the way to go.
But a good reman for basic transportation will get the job done.

Don't expect a big reman shop to do anymore than bore it
and drop in the same over size pistons.
They don't remove casting flash,double check pin fit or height.

I hope it all works out for you
 
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