Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Slow day… B25 pistons into an A50……??

So it’s a slow day, sat outside an airport hotel in Madrid and waiting for the time I can fly home… and I started wondering. And yep that can be pretty dangerous..

Dangerous for me is having hair brained schemes and dangerous for my wife is me following through with them and spending an obscene amount of money doing it.

So for a while I have been wanting to build an offset crank unit twin, I have a nice A10 crank on std and also a nice 74mm (A50/A65 crank, though it came out of an A50) and I have a junk A65 crank which can be used as a test piece. I recognise I’m no machine shop engineer either so trust others to do that work.. if they are prepared to do it and sometimes that can take a little while to get done. And That’s fine.

So some background, in the garage I have a 823 motor in an oif with A10 crank and std B44 pistons. I have a stock A65 motor in a pre oif which has a road race megacycle cam and raised the compression a few pips and enough bits to build a few other engines. I also have a couple of pairs of original high compression A50 pistons. Though I can’t remember what bore sizes those are and they have both had a life before, so I’m not desperate to use them. (The A10 crank would get used to make a psudo A70 with offset crank).

Then I started wondering, would a set of B25 pistons fit the 74mm crank?

.. I assume they have the 3/4 pin size same as the A65 and B44?
… and on standard they are +060 bigger than an A50 piston which would give 521cc. (I’m not bothered about the extra 21cc) but the stock barrel should be just fine.

Also new, quality A50 pistons of high compression are not available on the market so I’m thinking these would make a good alternative?

If you’re wondering why? at this point or why not build a faster A65 motor? then that’s a fair question. Short answer is I like having something different, the Stock A65 is quick enough and quicker still when I have resolved the head gasket leak and the 823’ well that was built purely for touring pleasure… and What would willingly be a similar bike (and I have spares for already) can have totally different characteristics just by changing a few things in the motor.

I always remember how well my friends A50 cyclone went with low compression pistons and spitfire cam. Though he probably broke most of the speed limits through town… I didn’t catch him up till I got to his house.

I have also wanted for a long time to build a replica A50 Daytona race bike so these are things that might come together in time.

So has anyone ever fitted a set of B25 pistons into an A50? And did you have problems with crown heights, dome shape or other issues?

Pin size on B25 is smaller, but get over that and you then have other options to look at to get higher compression.

C15 pistons

Trident/R3 pistons (I used one in a C15 to get higher compression).

And different bore size but used the B25 rod Tri Unit 500 pistons which were in 2 main varieties to cater to 2 head designs. Lots of different compressions to choose from.

A lot there I did not know and good food for thought. Thankyou Kommando.

To be honest I always thought that Triple pistons were almost identical to the B25 ones, like they were using up stock.

No, same bore but different heights above the pin, but close enough to the C15 to be able to be used and give a higher compression.

T100 is 69mm bore.

Thankyou again.

I think 69mm on the bore might be too much, but the R3 pistons look reasonable and can be bought for a decent price from MAP with good rings (about $150 plus shipping for all 3) the dome also looks fairly tall so there is room for some machining - potentially. The HC C15 pistons I think might be harder to find and when you do will likely cost a lot more.

At that price also it makes them reasonable enough to sell on if they are not right for my requirement.

When I get home I will look at pin centre to top of piston (before the dome).

I read someone once that Ed V was making A50 HC pistons but I’m sure he would list them if he did.

https://shop.alfamotive.nl/nl/ersat...k/kolben-buchsen/kolben-buchse-1600.html

I'm at the moment refurbishing a 1600cc AlfaRomeo Nord-engine and looking at 4 of these. 78mm bore should fit the oversize A65 block
don"t know the wrist-pin size but will measure tomorrow
regards Anne

Alfa pistons were a regular mod with the XS650-engines to increase engine size

I think the Alfa pistons have a 20 or 22mm pin from memory.
Nice pistons though.

Those are nice pistons and cheap as well, even without the liner. A little bit big though for my application.


Nick, I always appreciate your input. Positive or negative it’s always valuable info. What are your thoughts on the initial topic?

My initial approach would be.............
Measure up a trident piston first up. See what the pin height is like.
That will determine if and how much needs to come off the barrel.
You can always re-bush the rods for the smaller pins.

I just measured a T150 piston:
Center of pin to crown = 37.5mm
Center of pin to top of skirt = 19.5mm
Center of pin to bottom of skirt = 25mm
Pin diameter = 17.5mm (triples use no small end bush)

Of course, the Triumph head is different from BSA, with a higher crown to the pistons.