Piston interchangeability...

[Guy Wilson]

I recently had to drill both pistons out of a 1971ish Triumph trophy/adventure i found.  I paid about $ 100 for it, so I wasn't expecting great things of it. Its done about 6000 miles and other than the engine being completely solid and generally tatty looking, its not been opened up or had any metric spanners pointed at it in anger. anyway, my question. The pistons sticking out of my spare A10 bottom end looked very close to the ones I'd drilled out of the triumph. The triumph bore is 69mm standard, the A10 is about 70mm (I'm generalizing)  A friend of mine is running a 1940's 500 single motoguzzi on a Mitsubishi piston. Has anyone out there found any successful alternatives to the original factory pistons? What should I be looking at if I do find a Japanese alternative? Is the weight of the piston critical?
Guy
Kenya

[wardleybob]

Hy guy I have used datsun cheery pistons in my a10. but they are now hard to find now. Good luck. Wardleybob

[Flatboy 1950]

The standard Triumph bore is 71 mm.
Fitting pistons from a different make( or car engine ) can be a real headache.
Pin diameter / height from pin to crown / skirt length etc. will never be exactly what you need.
By the time you have made suitable small end bushes , shortened the skirt , and so on ...
you might as well buy the correct thing.
Another problem with this type of bodge is that nothing is usually recorded , causing more headaches down the line.

Regards , Flatboy.

[Briz]

Triumph pistons have a shorter compression height - the pin to top distance. Around 5mm difference from (very dim) memory. They also have deeper combustion chambers with wider valve angles so the valve cutouts & compression ratio would need attention as well.
I used 1275 mini pistons in mine as I used a Norton crank so needed a shorter height, but that wasn't an easy marriage.

[Guy Wilson]

Thank you all, Fatboy's accurate and unless I come across some NOS in Nairobi, which does happen, I'm better off ordering a new set from the UK,
All the best
Guy
Kenya

[BVSR]

Flatboy

The standard bore of 650 unit Triumph is 71mm but unit 500 (Adventure/Trophy) is 69 mm.

Regards Markku

[Flatboy 1950]

Yes , I never read the "Adventure" part.
I had a real Trophy , pre unit 1962, years ago.

Cheers , Flatboy.

[Beezageezauk]

As well as the Datsun Cherry, I was told that the old Skoda Octavia pistons were a direct replacement for the low compression A10 but stocks dried up years ago before I had chance to find out if it was true.

I don't suppose anybody on this forum has a reference book with the dimensions in so we can do a comparison??

Beezageezauk.

[Briz]

I wish there was such a reference book!
Years ago the Hepolite catalog was great, had all the specs. Long gone now.
The AE/Nural catalog has some data, but there isn't much in there that is applicable to an A10.
Why dont Wiseco/JE/venolia/Arias/alltherest publish this info online? there must be loads of blokes who are constantly putting weird engine combos together in their heads

[Guy Wilson]

In reply to Flatboy's post. Its not a real trophy..Its a quite an ugly looking bike in many ways with all the 70's stuff stuck on. I've seen a very nice version with a 19' front wheel rather than the 21 it came with. I think the frame is a BSA frame, possibly the same as the 350 single, I'm speculating though..

Thanks for all your thoughts on pistons. I'll probably by an original set from the UK. Its sound the easiest and least painful if I can ignore the cost...

Guy

[BSA_54A10]

Why dont Wiseco/JE/venolia/Arias/alltherest publish this info online? there must be loads of blokes who are constantly putting weird engine combos together in their heads

Because the instant they do some back yard hack in outer mongolia will use the info to start casting fake pistons from recyclede alfoil.
They will then flood the market and by the time  every A 10 owner had bought 10 sets that fell apart before they were run in ( because they are really really cheap ) the quality piston makers will be broke.

You have probably noticed a lot of catalogues that used to have good information in them are no longer published.
Now they just say suitable for A10.

[Guy Wilson]

I'm not advocating or encouraging the manufacture of cheap, pirated or substandard parts. Genuine NOS Replacement pistons bought out of the UK for a £100 will cost at least £250 by the time I get them in my hands in Kenya. I'm sure anyone else living a long way from the English Midlands will appreciate this. I was looking to find an alternative that maybe available locally. Kenya used to be a source of mainly British hardware until the 70's. its predominantly Japanese now. That's increasing true across many parts of the world and I thought there may be a good quality, probably Japanese, alternative that i could find locally.
riding old bike in this part of the world often requires lateral thinking...
Guy

[BSA_54A10]

It was not a criticism of you it was a statement of fact.
Conwire used to have a brilliant web site that listed every cable for every British bike made.
Some time after launching the web site their trade sales started falling off.
Investigations revealed some local morons using the data off the web to order very poor quality dirt cheap cables from India.
So they pulled the site which put an end to the expansions of the morons busines.
But the down side was that the market was now used to paying less for cables than the local manufacturing cost ans as such their motorcycle trade has just about ceased.

There are a lot of rubbish pistons out there that have been made to the on line catalogue specs so while the ring groves are the right size and in the correct place, the pin to crown height is correct but the wall thickness and exterior shape are all wrong. No matter what you do these will not work properly and a lot of them have broken around the oil ring grove.

[Triton Thrasher]

Flatboy

The standard bore of 650 unit Triumph is 71mm but unit 500 (Adventure/Trophy) is 69 mm.

Regards Markku

Misunderstanding is inevitable when people call it an "Adventure!"