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Piston Slapping
Posted by: The Stig ()
Date: November 12, 2020 03:59PM

I got the engine together on my '65 Evening Orchid vert earlier this year after a prolonged rebuild. Runs great but sounds like a diesel.

I decided it must be the 040 venolia pistons I installed. I got them used and thought these will be great and I dont have to buy pistons.

They do not have markings for direction that I can tell.

I recall not seeing which way was which but went with the combustion mark shadow - since these were used. Well I broke the engine down. I do not see them having an offset.

What is the typical offset?

I have a much larger set of venolias for a giant engine build. They clearly have an offset. Anyone familiar with these? Maybe they do not have any offset as machined. I know they are typically custom machined.

Not a big deal, I think at this point I will just get new pistons and keep these as wall hangers. Just curious more then anything.

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Re: Piston Slapping
Posted by: vairmech ()
Date: November 12, 2020 04:07PM

I'll take your Venolia pistons. I have 4 good ones that were in a race engine, 2 got damaged when a rod let go.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Piston Slapping
Posted by: thewolfe ()
Date: November 12, 2020 04:12PM

What was the piston to wall clearance?

Nate Wolfe
Portland OR
65 Corsa 180

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Re: Piston Slapping
Posted by: The Stig ()
Date: November 13, 2020 12:06PM

7-8 thousandths as I recall. A bit loose but I have run others that way.
I lean towards a bit loose vs too tight - I have had ones that started scuffing.

There is no offset to the pin on these after careful measurements.

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Re: Piston Slapping
Posted by: vairmech ()
Date: November 13, 2020 03:25PM

The Stig Wrote:
-------------------------------------------------------
> 7-8 thousandths as I recall. A bit loose but I have run others that way.
> I lean towards a bit loose vs too tight - I have had ones that started scuffing.
>
> There is no offset to the pin on these after careful measurements.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

.007-.008 is way more than what should be run, especially for a street engine and with pistons with no offset.

What I have found, just recently BTW, is even with thick cylinder wall and a piston that expands a little more than it should with a turbo .004" is all you need and is the max indicated for initial assembly.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Piston Slapping
Posted by: corventure Dave ()
Date: November 13, 2020 04:18PM

I seem to remember Corvair Underground sold pistons with no offset. I believe they were Harley Davidson pistons, if I remember right.
About 15 years ago I built an engine for a fellow who had bought all the engine parts from underground. I remember the pistons had no offset.
He insisted that I install them.
Met him again recently. Today he said the engine runs just fine. However it's a hobby car that gets driven only about a thousand miles each year, if that.

Corventure Dave

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Re: Piston Slapping
Posted by: jjohnsonjo ()
Date: November 13, 2020 04:33PM

They were KB Hypereutectic. I bought a set Lon let me return them because I was not happy with the lack of offset. They were also pre dimpled smiley for valve clearance and something wasn't correct with that either. They used Harley Rings.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Piston Slapping
Posted by: vairmech ()
Date: November 14, 2020 05:21AM

Keep in mind that these are Venolia pistons and they are race pistons. They worked just fine in my race engine with normal clearances although I couldn't hear piston slap if I wanted! LOL

Also keep in mind the clearances stated, any piston would have slap even with an offset.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Piston Slapping
Posted by: The Stig ()
Date: November 14, 2020 08:13AM

I had stated 'as I recall' and memory is fallible. I pulled more today and measured again. The spread is 3-5 thou. Still a bit loose.

I had likely convinced myself it was far more after several months back and forth questioning in my mind what had I screwed up.

I have felt the anguish of a piston seizing in a bore at speed and scuffing up. Many years ago and different circumstances, but still in my mind when I think about clearances.

Oh and the slap did quiet down above 750 RPM or so. In a standard trans car or even a closed car it would not really have been very noticeable - and the engine ran great! This is a convertible powerglide though so it was always there in drive at idle! In the end not really that big a deal. I will put in new pistons and cylinders. I fell for the 'I have them on and and they sound cool' train of thought.

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Re: Piston Slapping
Posted by: 67vair ()
Date: November 20, 2020 05:51AM

In the old days it was thought that installing the pistons "backward" to the pin offset was a way to decrease friction with a slight increase in power. I'm not going to debate whether this really makes any difference but I did this in an engine I built back in '77. I did not notice any piston slap and the engine is still running strong with about 30K miles on it. The pistons used were TRW 2206 installed at .003 clearance. So in reality it probably makes very little difference.
On another note, I used a set of Jahns cast pistons in an engine (not Corvair) and it sounded like a diesel. It ran fine but the noise was terrible, I never made that mistake again!

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Re: Piston Slapping
Posted by: jjohnsonjo ()
Date: November 20, 2020 08:30AM

Are these pin drilled or slotted pistons? I heard pin drilled make more noise.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Piston Slapping
Posted by: The Stig ()
Date: November 20, 2020 08:48AM

Pin drilled. These will go to Ken Hand for hopeful racing use where they belong.


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Re: Piston Slapping
Posted by: The Stig ()
Date: November 27, 2020 01:29PM

Just to close this out. Engine is back together and purring like a kitten with new pistons.
Ken the Venolias will be on their way to you next time I get to the post.

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Re: Piston Slapping
Posted by: American Mel ()
Date: November 27, 2020 01:59PM

I know I am strange, but I can't help but laugh every time I read the title to this thread.
Piston Slapping.
It sounds like a Martial Arts move or something.

"Yeah, there was a solid brawl going on out in the parking lot. But it stopped cold, when Joe stepped in and delivered a Piston Slap up the side of Hank's head!"

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
Have; '66 Monza Coupe - 140, 4-spd. Daily driver beater
'67 Monza Vert. - 140, 4-spd. Daily driver beater
'67 A/C Moredoor Monza
Have had; '61 Monza coupe, 80hp, PG
'62 Monza Wagon, 4spd.
'63 Spyder,
'65 Corsa
.
non-vair
'04 Dodge Cummins Quad Dually, approaching 400K
'03 Honda Del Sol
17'Terry

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