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Forged piston clearance
Posted by: gnrand ()
Date: January 23, 2018 03:26PM

140 motor that will use .040" Seal Power forged pistons (coated short skirts). The box calls for a min of .001" piston to cylinder clearance. This seems too tight to me so I decided to get some opinions.
Thanks
Jeff

Wildomar,Ca

1965 Corsa Hard Top
!965 Corsa Convertible
1968 Monza 140 Convertible 4 speed
1966 435 HP Crown
1986 Buick GN

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Re: Forged piston clearance
Posted by: MattNall ()
Date: January 23, 2018 03:44PM

Many will scream I always used 0.006" 0.003 all around... for street...

Admittedly this was 40+ years ago.





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Re: Forged piston clearance
Posted by: Chilly Willy ()
Date: January 23, 2018 04:38PM

Yep, been down this rabbit hole. I believe it’s .001 per side,meaning .002 overbore.




The Corvair, Keeping Nader Notorious winking smiley


Will
Northern California

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Re: Forged piston clearance
Posted by: Frank DuVal ()
Date: January 23, 2018 05:53PM

I used .004" (diameter over) on the last 145 cu in I built with a fellow club member. This was reusing the .020 pistons that were in the engine. Runs great!

Frank DuVal

Fredericksburg, VA

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Re: Forged piston clearance
Posted by: 66vairman ()
Date: January 23, 2018 06:07PM

Chilly Willy Wrote:
-------------------------------------------------------
> Yep, been down this rabbit hole. I believe it’s
> .001 per side,meaning .002 overbore.

This has come up before and I agree with Chilly.

The old TRW forged piston specification was 0.0015" and the grammar on the TRW sheet suggest that is for ONE side piston to wall clearance - OR a total of 0.0030"

I talked with the Bruce at Engine Machine who does a lot of Vair machining work and he concurred. Cylinders honed to between 0.003-0.004". Did two engines like that - both run fine.

And to add ---- I pulled apart an engine I bought with scuffed cylinders and the forged pistons total clearance was 0.0010" to 0.0015" - too tight.

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Re: Forged piston clearance
Posted by: vairmech ()
Date: January 23, 2018 06:14PM

You told us what pistons you were using but what cylinders are you using and what engine are you building?

The piston alone does not dictate what the clearance should be.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Forged piston clearance
Posted by: cooterr ()
Date: January 23, 2018 06:32PM

I had the same problem. I bought the new Clarks forged pistons made by primo and that data sheet that came with the pistons said basically the same thing. I gapped mine at .002 and scored three of them in the first thirty minutes of runtime on the engine. I have since gapped them at .004 and have had no problems.

I think the problem arises when we look at the speck sheet and it does not specify a difference between water and air cooled applications. Their spec sheet I would bet is for water cooled engines.

Buyer beware

Tom Miller
Performance Corvair Group
Circle City Corvairs
Corsa

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Re: Forged piston clearance
Posted by: 63turbo ()
Date: January 23, 2018 07:15PM

Just use .004" clearance and forget it, no scuffing no noise and no problems. All of my stock bore stock clearance cast pistons have all had deep scratches in the skirts, the last set I've used and inspected had 30,000 miles on them and pretty much looked like new pistons except for one that had minor scuffing only on one side and was probably caused by gas wash down from a bad exhaust cam lobe. The biggest difference between that set of pistons and all of the others that had massive scratches that I can think of was that they were purposely set a bit looser than recommended- .003-.0035 as I recall. Forged should be somewhat looser than cast ones.

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Re: Forged piston clearance
Posted by: gnrand ()
Date: January 23, 2018 09:52PM

vairmech Wrote:
-------------------------------------------------------
> You told us what pistons you were using but what
> cylinders are you using and what engine are you
> building?
>
> The piston alone does not dictate what the
> clearance should be.

Stock cylinders that are bored 40 over. Engine is a street 140 with a Schneider 270 cam.
Yes, I will pay a visit to Bruce.
Thank you

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Re: Forged piston clearance
Posted by: vairmech ()
Date: January 24, 2018 05:12AM

gnrand Wrote:
-------------------------------------------------------
> vairmech Wrote:
> --------------------------------------------------
> -----
> > You told us what pistons you were using but
> what
> > cylinders are you using and what engine are you
> > building?
> >
> > The piston alone does not dictate what the
> > clearance should be.
>
> Stock cylinders that are bored 40 over. Engine is
> a street 140 with a Schneider 270 cam.
> Yes, I will pay a visit to Bruce.
> Thank you
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

With that you can get by with .003" clearance as long as you are careful during the break-in time. If you want to play a little harder, which most do during break-in, then I would use .00035 clearance.

You really need the piston measurements and that needs to go to the person boring the cylinders and they can stop the hone process at the specified size needed.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Forged piston clearance
Posted by: gnrand ()
Date: January 24, 2018 06:09AM

Yes, I will take the pistons, rings and the cylinders to the machinist. The rings are Grant and the top and second are the same (PN 2242.040). Recommended hone is 220 to 280. What end gaps do you like to run.
Jeff

Wildomar,Ca

1965 Corsa Hard Top
!965 Corsa Convertible
1968 Monza 140 Convertible 4 speed
1966 435 HP Crown
1986 Buick GN

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