Explain wrist pin offset...

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acarlson
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Explain wrist pin offset...

Unread post by acarlson » Mon Jun 26, 2017 10:34 am

...Particularly how I can measure it...

Alec
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Re: Explain wrist pin offset...

Unread post by 66vairguy » Mon Jun 26, 2017 12:13 pm

Lots of explanations online. Basically since combustion typically starts before TDC. the wrist pin center is offset to reduce piston rocking motion and rate of rocking motion which when excessive can cause cracked or broken pistons

I've heard Corvair engines with the piston offset 180 degrees out - they sound like thrashing machines and it's only a matter of time before pistons start to crack or break.

There is confusion about what is the "FRONT" or "REAR" of a Corvair engine. Now most Corvair piston instructions reference the flywheel for direction to orient the piston direction mark.

NOTE: in reverse rotation Corvair engines the piston offset DOES have to be 180 degrees out.

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acarlson
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Re: Explain wrist pin offset...

Unread post by acarlson » Mon Jun 26, 2017 12:51 pm

I'm really looking for the answers to 3 questions:

How much offset is there ?

Can you measure it with calipers ?

Which side of the piston has the offset (front/flywheel or rear/balancer) ?
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Re: Explain wrist pin offset...

Unread post by bbodie52 » Mon Jun 26, 2017 6:14 pm

Brad Bodie
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acarlson
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Re: Explain wrist pin offset...

Unread post by acarlson » Mon Jun 26, 2017 7:41 pm

Thanks Brad.
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Re: Explain wrist pin offset...

Unread post by rfw66 » Wed Jun 28, 2017 8:54 am

Great reading!
Anyone have experience with the Keith Black pistons sold by Lon at Corvair Underground?


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Re: Explain wrist pin offset...

Unread post by thewolfe » Wed Jun 28, 2017 9:26 am

I have a set in my 65 turbo and that motor has taken some serious abuse. Overheated to the point of shutting down, pistons banging on 4 different dropped valve seats over the years, many instances of engine knocking from too much boost. I've been running 15 psi with a 40dcoe the last 4 years or so and I have a lead foot. Have a qjet now and it still runs good with those same pistons and rings that I haven't touched and were in the car when I bought it. I'm really surprised they have held up as long as they have. Don't think you can get them anymore though. A good set of forged pistons should take just about anything you can throw at them.
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Re: Explain wrist pin offset...

Unread post by rfw66 » Wed Jun 28, 2017 12:02 pm

I was asking about the super hypereutectic alloy pistons that in Lon's description he says, "We asked them to make a special piston for the Corvair....."
In the pictures on his site, there doesn't appear to be any off set in the pin boss. Any feedback on these?


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Re: Explain wrist pin offset...

Unread post by bbodie52 » Wed Jun 28, 2017 1:36 pm

ImageImage

KEITH BLACK PISTONS!!
No one has ever offered a piston of this caliber for the Corvair!
:link: http://www.corvairunderground.com/govair/kb.htm

Image

Image
Corvair Piston Talk
:link: http://corvaircenter.com/phorum/read.ph ... 714,915720
Hemmings wrote:Silv-O-Lite/Keith Black also offers hypereutectic pistons for stock and mild performance applications. Their KB Silv-O-Lite series piston features reinforced top lands for added strength and a proprietary diamond-finishing process on the piston skirts. After the finishing process, the piston skirts are coated to reduce cylinder-wall scuffing.

Keith Black also offers KB Signature Series performance pistons for Ford, Chrysler and GM muscle car applications. Made from hypereutectic 390 aluminum alloy, Keith Black Signature Series pistons are CNC-machined for accuracy and are heat-treated with 16 to 18 percent silicon. Each Signature Series piston features full-floating wrist pins and precision-drilled oil returns and pin oil supply holes. The unique T-6 hardened hypereutectic 390 material is 30 percent stronger than cast-aluminum. The 390 material is also a reflective alloy that the company claims will increase engine power by 2 percent over other hypereutectic products, by reflecting heat back into the combustion chamber.

Keith Black pistons incorporate a higher top ring groove, which better stabilizes the piston assembly within the cylinder to prevent wear and allows for a tighter piston-to-cylinder fit. Piston crowns and valve reliefs are also CNC-machined and diamond-turned ring lands are reinforced for higher strength.

Because Keith Black-style pistons place the top ring up higher on the piston, and the pistons make more horsepower by reflecting heat energy back into the combustion chamber, the top ring runs hotter and more end gap is required than on a conventional piston. Increasing ring end gap does not affect performance or oil control because end gaps will normalize once the engine reaches operating temperature.
:link: https://www.hemmings.com/magazine/mus/2 ... 98931.html

:BradBodie:
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Re: Explain wrist pin offset...

Unread post by rfw66 » Wed Jun 28, 2017 2:41 pm

Although all the links were good information about hypr pistons and their good and bad characteristics, not one person addressed piston pin offset with the KB pistons!
Without proper pin offset my hunch is that they will have piston slap all the time.


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toytron
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Re: Explain wrist pin offset...

Unread post by toytron » Sat Jul 22, 2017 4:32 pm

Alec,

To answer your questions;
I will try to find the offset for you.
I don't think that you could accurately measure it with calipers.
And finally like the first question. I would have to find that info also.
My suggestion would be to give Clark's a call because they have had to investigate this info or at least the company that they buy from would know those answers I am certain.

Oh and the offset would not be front or back but up or down. The mark on the piston is to make sure that whatever side you put it on you by going off of the mark facing the front will have just orientated correctly. I hope that wasn't too wordy.

Ed Stevenson


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Re: Explain wrist pin offset...

Unread post by toytron » Sat Jul 22, 2017 4:55 pm

Alec, I came across this picture that davemotohead posted on the other forum of a piston where you could easily see the offset. Image

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Re: Explain wrist pin offset...

Unread post by WinginEngineer » Sat Jul 22, 2017 8:35 pm

Trig.

Take that picture from Ed, measure each side wall of the hole, subtract them and divide the difference by two. This will give u the "opp" measurment.

Now take the distance of the rod from the center of the rod bearing to the center of the wrist pin hole and that becomes your "h".

The sine of the angle (offset) is opp ÷ h. Therefore the arc-sine of opp ÷ h is the angle of the offset.

If you've never done trig the calculator should read like so: sin^-1(opp/h).

Now i have no clue what the offset angle should be, and it won't be a perfect measurement because of casting imperfections and human error in measuring, but it will get you close enough.

Hope that helped even a little.
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Re: Explain wrist pin offset...

Unread post by rfw66 » Sun Jul 23, 2017 7:30 am

Trig aside....Still not a definitive answer on pin offset in the KB pistons! Anyone?
The offset is obvious on every other Corvair piston I've ever looked at, but can not be seen on the KB pistons. Why is that


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Re: Explain wrist pin offset...

Unread post by WinginEngineer » Sun Jul 23, 2017 9:59 am

The offset is there, in part, to slightly lift the piston during the stroke to counteract gravity and prevent the bottom of the cylinder and piston from wearing more than the top causing an oval cylinder.

That is why you see them on ALL corvair pistons. The physics of the engine are what they are.

One of earlier posts you said they told you "we asked tgem to make a special piston for the corvair".

GM would not have likely made one just for the corvair. They would have used a casting as close as possible and the differences (IE offset) would have been handled in machining, hence the visible offset in comparison to the casting.

If the KB pistons WERE designed exclusively for the corvair, it is very possibe the casting/forging was too, meaning the offset would have been put in at that level as well.

Looking at the casting from the inside 0.050 or 0.060 is easy to see. From the outside it would not be.

Therefore, without having a set in hand to measure, you would either need to find someone who does or ask the manufacturer.
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Re: Explain wrist pin offset...

Unread post by rfw66 » Tue Jul 25, 2017 6:55 am

Ed, I the piston you show from a dave' post on the other forum, does on look like a KB piston. Can you give a link to motohead's post


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