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Martin Southwell Samba Member
Joined: November 24, 2010 Posts: 984 Location: Bath, England
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Posted: Mon Apr 01, 2024 2:56 pm Post subject: The quest for replacement 25hp pistons |
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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? |
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johnshenry Samba Member
Joined: September 21, 2001 Posts: 9364 Location: Northwood, NH USA
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pastellgreen Samba Member
Joined: January 06, 2012 Posts: 1048 Location: Germany
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Posted: Tue Apr 02, 2024 3:18 am Post subject: Re: The quest for replacement 25hp pistons |
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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. |
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Martin Southwell Samba Member
Joined: November 24, 2010 Posts: 984 Location: Bath, England
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Posted: Tue Apr 02, 2024 4:34 am Post subject: Re: The quest for replacement 25hp pistons |
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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.
Last edited by Martin Southwell on Tue Apr 02, 2024 9:53 am; edited 1 time in total |
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esde Samba Member
Joined: October 20, 2007 Posts: 5966 Location: central rust belt
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Posted: Tue Apr 02, 2024 6:00 am Post subject: Re: The quest for replacement 25hp pistons |
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You know, a 74mm crank would solve the 5mm piston crown problem _________________ modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that. |
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Martin Southwell Samba Member
Joined: November 24, 2010 Posts: 984 Location: Bath, England
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Posted: Tue Apr 02, 2024 9:55 am Post subject: Re: The quest for replacement 25hp pistons |
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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? |
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esde Samba Member
Joined: October 20, 2007 Posts: 5966 Location: central rust belt
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Posted: Tue Apr 02, 2024 6:32 pm Post subject: Re: The quest for replacement 25hp pistons |
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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 _________________ modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that. |
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heimlich VWNOS.com
Joined: November 20, 2016 Posts: 6593 Location: Houston, Texas
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Posted: Wed Apr 03, 2024 4:07 pm Post subject: Re: The quest for replacement 25hp pistons |
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Can you cut a set of connecting rods and weld them together adding 5MM? _________________ www.vwnos.com [email protected]
Classic Brands. Classic Quality.
Not all parts are made the same. NOS OE/OEM parts made mainly in West Germany, Early Germany, and Early Brazil are where VW produced the best quality parts and best fitting products.
5% Off your order with coupon code: 5%OFF
Restored Distributors Available (<--Click here) |
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pastellgreen Samba Member
Joined: January 06, 2012 Posts: 1048 Location: Germany
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Posted: Thu Apr 04, 2024 8:13 am Post subject: Re: The quest for replacement 25hp pistons |
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Martin Southwell wrote: |
I've heard that people have done this, but must leave the cylinder wall mighty thin. |
Last step oversize for 25hp cylinder is 76,020. The first step for 36hp is 76,990. The difference is 0,97mm in diameter, the cylinderwall will then decrease about 0,485mm.
I would still call it a possible way. 36hp sets are available everywhere, of course for some money. I would use the pistons and sell the cylinders.
Or you search for a set of good used 36hp pistons for reasonable money and let the 25hp cylinders match the used pistons. Of course with new piston rings. |
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esde Samba Member
Joined: October 20, 2007 Posts: 5966 Location: central rust belt
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Posted: Thu Apr 04, 2024 12:35 pm Post subject: Re: The quest for replacement 25hp pistons |
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pastellgreen wrote: |
Martin Southwell wrote: |
I've heard that people have done this, but must leave the cylinder wall mighty thin. |
Last step oversize for 25hp cylinder is 76,020. The first step for 36hp is 76,990. The difference is 0,97mm in diameter, the cylinderwall will then decrease about 0,485mm.
I would still call it a possible way. 36hp sets are available everywhere, of course for some money. I would use the pistons and sell the cylinders.
Or you search for a set of good used 36hp pistons for reasonable money and let the 25hp cylinders match the used pistons. Of course with new piston rings. |
The right machinist could turn 36hp pistons down a bit, so the cylinder walls were not danger thin. Of course that would mean ring grooves too, and finding rings to fit. _________________ modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that. |
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Martin Southwell Samba Member
Joined: November 24, 2010 Posts: 984 Location: Bath, England
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Posted: Thu Apr 04, 2024 2:50 pm Post subject: Re: The quest for replacement 25hp pistons |
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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. |
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djfordmanjack Samba Member
Joined: June 15, 2009 Posts: 2171 Location: Graz, Austria, Europe
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Standardfahrer Samba Member
Joined: December 30, 2009 Posts: 42 Location: Germany
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Posted: Fri Apr 05, 2024 1:27 am Post subject: Re: The quest for replacement 25hp pistons |
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I have fitted Polo/Derby pistons bought by E.P. Hagen. I'm running them since 20years.
Cheers Marcus |
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Martin Southwell Samba Member
Joined: November 24, 2010 Posts: 984 Location: Bath, England
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Posted: Fri Apr 05, 2024 2:26 am Post subject: Re: The quest for replacement 25hp pistons |
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djfordmanjack wrote: |
Martin did you make use of the Stahlwille ring compressor tool?
cheers Günter |
Not yet! |
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