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350 Piston interchangeability


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Hi, this is my first post here. I have most of an M116 350 Alpina which I got for $100 20 years ago and I have been slowly gathering up some of the missing parts. The main things I still need are a piston and con rod kit. While pistons are available new, they are quite expensive. Occasionally on ebay I see listings for NOS pistons from other models such as 360 Astro, Frontera, etc.  Are any of these suitable? I believe I am currently at standard bore but as the cylinder is quite rusty it will likely need a rebore in any case. Alternatively are there any more common Japanese model pistons which can work? Probably I should just save up and get the proper thing, but any suggestions are welcome.



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My model 138 Alpina (engine very similar or identical to model 116) has what I think is a 360 Pursang piston which is not exactly right for the model 138 Alpina but still works great. The Pursang cylinder has a bridged exhaust port so this piston I'm using has holes through the front to lubricated the bridge. The 350 Alpina has an unbridged exhaust port so shouldn't have holes there.

The other difference is that Pursangs have additional transfer ports that work in with other holes in the piston and so those piston holes are superfluous in the Alpina engine.

Some Bultaco 360/370 pistons might have a shorter rear skirt than the 350 Alpina but from memory the Pursang piston in my Alpina had the same skirt length as the piston that came out. My memory is not that certain though as it was over 20 years ago that the model 138 motor was rebored and I haven't had it apart since then.

I don't know of any other brand bikes that use a piston that can be used in a 350 Alpina.

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Thanks, that is really useful information. I had noticed those holes you mention in photos of Pursang pistons but wasn't sure of the purpose, and I don't have an Alpina piston to compare to. From the sounds of it it would work but maybe would result in more mixture being blown out the exhaust port. I should wait for the proper thing I suppose. What about Sherpa pistons though, or do they have a different compression height due to the shorter stroke?

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I presume you been in touch with Hugh’s Bultaco and Ralph’s Bultaco Parts US?

Ralph, bought all of Jan Mobley’s stock.

I believe Hogan Cycle Parts have stopped taking orders since February. I have no idea if they plan to restart, or sell their stock on to someone else.

Edited by model80
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