The quest for replacement 25hp pistons

[Martin Southwell]

As the supply of 25hp barrels and pistons doesn't improve over time, I've always kept half an eye open for alternatives to fit to 25hp barrels, even though, luckily, I have enough NOS sets for my own use. One I've had for years, was offered for sale at a large steam rally, and when I saw the box with 75mm on it, I thought it would be beyond my price, but casually asked the vendor what car they were for. 1200cc Beetles I was told, and so I asked if they were for any specific year? All years I was told! I bought them at a more than reasonable price, with the vendor no doubt thinking I was a mug, and with me knowing I wasn't!

Anyway, most of us have heard that early Polo pistons could be fitted, although I've never come across anyone who has fitted those, and then by accident, 10 years ago, I came across some Opel pistons in Greece, which I felt would fit (and do) and so bought 3 sets. One of these can be seen in the right hand barrel. I was later told this was a German held secret, which the Germans had known about for years, and had fitted to their cars.

Needless to say, the supply of these have long since dried up, and so I've kept up the search, and recently found a weird source of non automobile pistons, from another part of the world, which I've taken a gamble on. The result can be seen in the picture. They fit well, and even have the same diameter piston pin, and I had calculated that I would need to cut down the piston crown a little, but it turns out that they are 5mm too short! (Seen in the left hand barrel) Looks greater in the picture, as not pressed fully into place. Shame you can't weld a layer on the top? I wonder what the compression ration would be if used? Any suggestions?

[johnshenry]

I have a set for sale:

https://www.thesamba.com/vw/classifieds/detail.php?id=2637739
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John Henry

'57 Deluxe
'56 Single Cab

[pastellgreen]

and what about increasing the bore to 77mm? The last step oversize isn't that far away from the first 36hp measurement. One could reuse the original cylinders and use 36hp pistons.

[Martin Southwell]

I've heard that people have done this, but must leave the cylinder wall mighty thin. Plus, where do you get a set of 30hp (US=36hp) pistons from these days? Not going to be cheap wherever you get them from. My original idea was to offer these pistons for sale at £300 a set, including rings & pins, thereby making rebuilding of a 25hp engine more viable.

[esde]

You know, a 74mm crank would solve the 5mm piston crown problem
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modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that.

[Martin Southwell]

Now there's a clever idea! Know of any supplier? What are the compression ratios of 25hp engines, especially those with flat, and domed pistons?

[esde]

https://www.wolfsburgwest.com/cart/DetailsList.cfm?ID=111198600A
I believe a stock 36hp is something like 6.6:1.
Bumping the stroke to 74 would put it over 7.
But once you change the crankshaft, what's the point of trying to keep it 75mm bore? Just use a set of p/c from a 36hp
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modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that.

[heimlich]

Can you cut a set of connecting rods and weld them together adding 5MM?
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[pastellgreen]

[esde]

[Martin Southwell]

Pistons are actually tapered, being narrower at the crown than at the base, so machining would be out. I'm personally not short of barrels & pistons (Have 3 NOS sets), but as a challenge, have been looking to see what alternatives can be found for 25hp engines, and thought I had cracked it! They could still be used, and may work out to have a similar cylinder volumes to dished pistons, but need to do more research re compression ratios. I'm still amazed that I found something very similar in an unexpected place.

[djfordmanjack]

Martin did you make use of the Stahlwille ring compressor tool?
cheers Günter
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Hotrods, Fords, Veedubs and Triumph Twins !

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[Standardfahrer]

I have fitted Polo/Derby pistons bought by E.P. Hagen. I'm running them since 20years.

Cheers Marcus

[Martin Southwell]