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Piston to Cylinder Clearance; Piston Clearance
Topic Started: Feb 5 2013, 12:13 PM (2,753 Views)
High MX
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The block I am using for rebuild has significant wear on the cylinders and I am curious as to what is the max clearance between the piston and cylinder wall before I have to go to oversize pistons? :news
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PTA2PTB
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I'm totally awesome! I swear.

Keep in mind that the "piston to cylinder wall" clearance isn't your only consideration; you need to make sure there's no (appreciable) ovality.
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Old Man


If it shows "significant wear" then be sure and check for out of round and taper as well as size. I am not sure what the clearance is without taking the time to look it up but I know it is surprisingly close tolerance.

If it were mine and it showed "significant wear" and I had the time and monies I would definately bore it.
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Deleted User
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The Factory Service Manual limits for cylinder bore taper and out of round is 0.0039".

I don't overhaul an engine without boring oversize if the out of round/taper is no more than 0.0025". You will still be able to see the factory hone marks.

The new limits for piston to cylinder wall is 0.0008" - 0.0015".

I use Top-Line oversize pistons, P/N PASZ-1L 020. They always measure 2.9319" at 15mm above the bottom of the piston on the skirt, which is the factory new minimum size for 0.020" oversize pistons. They come out of the box balanced within 1 gram or less, usually less.

http://toplineauto.com/ Click on locations and call on the phone.

Take the block, pistons, and rings to the machine shop, along with a Factory Service Manual. Explain what clearances you want. I had a conversation with a GMF member yesterday who had a block at a machine shop. The guy wanted to bore the cylinders to 0.0020" piston to wall clearance. This is worn out, brand new. No bueno. :shake .

I have my blocks bored to 2.9327" - 2.9329". The top compression ring will gap at 0.008" - 0.010", which is new limits.

The other piston brands that I have experience with are DNJ/Rock, and Clevite/Perfect Circle. Both of these brands have dimensions that are less than 2.9319. None of the pistons were the same size, and the mass varied by as much as 2.5 grams. They will still work, but you'll have 3 different size cylinder bores, and have to do quite a bit of carving on the pistons to get them to balance.

Don't forget to have the block decked off a little bit.

GG
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Coche Blanco
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Also, it's always good practice to have the pistons measured BEFORE you have the shop bore and hone the holes.
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Coche Blanco
Feb 5 2013, 01:46 PM
Also, it's always good practice to have the pistons measured BEFORE you have the shop bore and hone the holes.
:gp



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Smidgen
Smidgen

Geo Glenn
Feb 5 2013, 01:38 PM


The new limits for piston to cylinder wall is 0.0008" - 0.0015".

Take the block, pistons, and rings to the machine shop, along with a Factory Service Manual. Explain what clearances you want. I had a conversation with a GMF member yesterday who had a block at a machine shop. The guy wanted to bore the cylinders to 0.0020" piston to wall clearance. This is worn out, brand new. No bueno. :shake .

I have my blocks bored to 2.9327" - 2.9329". The top compression ring will gap at 0.008" - 0.010", which is new limits.

GG


I recently overhauled my engine. I got it overbored, and I now realize what the guy was saying when he took "20" off instead of the "150". He didn't talk to me before he did it. He just told me when I had my block back... It runs great. It has blow by at 75mph, but I was just figuring the rings were still seating, but now I am thinking otherwise. I only have 5k on the new engine. Anything I should really be worried about?
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GeoPete
AAS Auto Technology past ASE Master Tech

Broken rings.
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Smidgen
Feb 5 2013, 02:20 PM
I recently overhauled my engine. I got it overbored, and I now realize what the guy was saying when he took "20" off instead of the "150". He didn't talk to me before he did it. He just told me when I had my block back... It runs great. It has blow by at 75mph, but I was just figuring the rings were still seating, but now I am thinking otherwise. I only have 5k on the new engine. Anything I should really be worried about?
It will continue to oil fart. The only way to fix it is to get another block and have it properly machined.

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