I've just collected an A50CC project - U.S. import and all in bits but looking largely complete.
I'm not yet sure if this was a full competition model or the road legal variant. So far all I have to go on is that the rear hub has a notch for a speedo gearbox drive tag (but there's no speedo gearbox with it) and it has an alternator which I think is the 5 wire type.
The pistons are stamped 68 506 and then 30 4/65M
A quick measure of the piston diameter just below the rings with a vernier shows 65.30mm
It will be a while until I get around to the rebuild as I have other bikes on the go but I'm keen to find out what these pistons are - I haven't been able to find any good references using that part number.
Grateful for any info.
Cheers,
Bernie
https://www.baxtercycle.com/FLIPPING%20BOOK/1965_A50_A65/6/index.html#zoom=z
going by the parts book for 1965 A50 the 10:1compression piston was 68-0505, +.020" (0.5mm) was 68-0507 and +.040" was 68-0509.
going by that I would say 10:1comp and +0.010".... +0.030" would be 68-0508
Im thinking that although +0.010 and +0.030" was available , to cut down on stock held only the +20 and +40 was stocked. having had many rebores done for compressor work and my own bikes, often 0.010 does not fully clean up which is why my machinist always insists he bores first and then I get the piston to suit and he finishes off. he has been left with pistons that customers have bought for him and on boring he has had to order the next over size.
Pistons are 10;1.
Below is piston list fromthe parts book.
The number on the crown is the number of the bare piston. The number in the book, one lower. is for piston complete with rings etc.
Many thanks Chaz and Julian, when I've checked the condition of the pistons and bores I'll know if I have the option of keeping it as is at 10:1.
If not I have the difficult decision of whether to go for something a little gentler for road use - or maybe not :D
Cheers,
Bernie
I have +.030" on my Royal Star although the pistons have the part no. 68-439 which isn't listed in the parts book but they also have 1.00 m/m oversize stamped on the top, which as we all know is wrong as 1 mm is 0.039".
They are also date stamped as August 1972. As this 0.030" bore is nigh on perfect, and the pistons show signs of being seized at some point, my guess is they are second hand and previous owner had to get what was available, or go to 0.040"
I would recommend 9:1 pistons as the motor is far more user friendly for what we want these days.