AA Piston Deck Height Different Than Stock

[keysas]

Hi all - literally my second post to TheSamba. I am doing a 40hp rebuild and have recently received a set of AA 77mm pistons. I know you can go 83mm on this engine, but I'm doing all original stock.

Here's my problem - the original pistons that came off of the engine had a deck height of about 0.050 inches (1.27mm). The new AA 77mm pistons have a deck height of about 0.092 inches (2.3mm)! When comparing new to old, the distance from the top of the link pin hole to the top of the piston, there is a full 0.040 inch (1.0mm) difference. I thought these new 77mm pistons would be a direct replacement with no additional machining.
Has anyone else noted this difference? I'm guessing 0.092 is too deep for reasonable deck height on this already under-powered engine. Should I get the cylinder tops machined down by 0.040 inches? Is this info covered elsewhere in the Samba forums? Lots of questions.

[jsturtlebuggy]

I never used A&A for a 40hp so never had that problem.
But with other A&A and Mahle cylinders I have had to cut cylinders down to remove deck height.
It best to cut the bottom of cylinder where it slips into case.
And yes there are threads about other having to cut cylinders.
Glen who goes by Modok is a machinist by trade and I would ask him if you need more info.
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[ps2375]

nvm, I had read it wrong.

[campingbox]

Yes, you can remove material from the cylinder steps to lower the deck height. You can also machine the side of the case.

Measure the head volume before doing anything, you may be happier with the compression ratio now than you will be after cutting off .040".

[GTV]

The usual solution is to install a stroker crank.


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[mark tucker]

[keysas]

All:
Thanks for the replies. I have measured the volume of the head combustion chamber and did a bit more research. The specs are:
piston bore: 76.95mm
piston stroke: 64mm
deck height: 0.082in or 2.08mm (no paper gasket at engine/cylinder interface)
combustion chamber volume: 44mm^3
...and using the CB Performance calculator...
Compression ratio = 6.5

A CR that is lower than stock, but its all due to that large deck height that will be difficult to reduce without flycutting the head or machining the cylinders.

So here's the mystery I am working on:
I think the original 77mm pistons on my 40 hp engine had a compression height (distance from center of wrist pin to piston surface) of 39.2mm. That's what I measured on the old set.
The AA Pistons I purchased to be a direct replacement for the stock 77mm pistons on my 40 hp engine have a compression height of 38.05mm.
And here's the weird part - the AA Pistons that are "big bore" at 83mm for a 40hp engine have a compression height of 39.2mm! The same as the original 77mm set!
Why would the new 77mm set be shortened by over a mm? This is where my deck height is getting hosed up! While it will not help my situation, any insight would be interesting.

[Dale M.]

Wrong piston cylinder set? .... Stroker pistons?

Dale
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All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[modok]

According to my spec sheet
111 198 057 has 38mm pin height, 101 length cylinder, fits head bored 84mm

111 198 057A has 39mm pin height and 102 length cylinder fits head bored 90mm

Which kit is this supposed to be?

[mark tucker]

stroke it 4mm or more.

[keysas]

I am posting on what I learned during this process, just in case there is someone else out there experiencing the same issue.

I contacted AA and asked why the stock 77mm pistons have a shorter compression height than the big bore 83mm pistons and my original stock 77mm pistons. The answer is quoted here:
"We are aware of the AA (77mm) pistons being a little smaller, some time ago we were asked to do that because 90% of the cases and heads have been cut down.. People were trying to make up for that without having to shim the engine."

I considered all the suggestions made previously on how to fix this issue - flycutting the heads, stroker crankshaft, longer rods, different pistons, etc. I even asked a machine shop about milling the cylinders down by 0.040 inches. In the end, I have decided to just live with the extra deck height and see how the engine runs. This will not be a daily driver and with a deck height of about 0.082 inches, I get the calculated compression ratio of about 6.5. Besides, if you start machining the cylinder height or flycutting the heads, I'm not sure how you account for the modified heights when you try to get the intake manifold to fit across the engine between heads. Mine fit perfectly since I did not do any modifications.

Again, thanks for all the help! I am learning TONS as I work through these issues.

[VWCOOL]

are the installed heights of the barrels the same as standard? ... and the pistons have (effectively) less crown height? Or are the barrels longer and intended to be installed to a decked case?

[campingbox]

I don't think you would even notice .040" per side taken off the barrels when it came time to fitting the manifold. That's considerably less than 1/16" per side. If the intake manifold needed it, you could pretty much bump it against the shop floor to move it in a 16th or so. If that didn't do it, stick a few screwdrivers in and bend it inwards a little bit.

[the2ndcashboy]

How much did the machine shop quote you to trim the cylinders? If more than 100$ I'd shop around and find someone cheaper. The first machine shop I asked about mine wanted 500$ I think I ended up paying like 50$ for a VW shop to do it. I think your motor would be a lot happier with a bit less deck height/more compression.
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[mark tucker]

I would go with longer rods if possiable and shim what needs to be shimd....if the cutting will cost the same or more or if you are using used oe rods. if you have your own lathe to do it than thats a no brainer.....just use your brains when cutten.

[[email protected]]

What Mark said; Simply use a 69mm crank. If you are going to be off .050", it may as well have some benefit! This also allows the use of the stock cylinders (unmachined), with some cheap cylinder shims to finish it out.
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[Mr. Motorhead]

Here's what happened. AA performance first made 36hp 77mm P/C sets. They then wanted to fill the market gap on 40hp and made a stock 77mm and 83mm big bore kits. Both 77mm pistons (36hp/40hp,same size, same rings) are exactly the same save for original factory 40hp pistons being .040" taller. The pistons they used for the 77mm 40hp set? The same pistons they used for for the 36hp sets. Why make two pistons when one will do was the thought. It is true that most (not all) 40hp heads and cases will have been decked or flycut at some point. You could have your decks evened up and flycut the heads and be pretty close to stock static compression.