Original Sunbeam pistons are thin on the ground and it seems that the rings to go with them are scarce too. The 1930 Model 90 was fitted with a 7.5 : 1 piston of Australian manufacture. This works well enough but is around 80g heavier than an original. We suspect that this accounts for the increased vibration that is apparent when compared with the Dodson bike or any of Archie’s for that matter. It’s really not a big deal but as we had a spare 7:1 Sunbeam piston and cylinder left over from the 1928 rebuild we thought we’d try it out for the summer – should we have one this year.
The bore of our spare cylinder although worn was deemed serviceable and after a light hone we sourced a set of three plain NOS compression rings. A stepped oil scraper would often have been used in the lower position but none were available. The top ring height was 2.35mm whilst the other pair were 2mm but the ring gaps were huge running from 0.015″ to over 0.020″.
Ever helpful FW Thornton located a brand new set of 80mm rings which were supplied un-gapped and with a stepped scraper to boot! Happy days, until I tried to slip the piston into the bore. Just as I reached for the mallet to help things along I realised that the radial depth of the new rings was too large for the piston. Sure we could increase the ring depth of the piston but there’s not a lot to play with and we really don’t want the top to come off.
Peter Woodward came to the rescue with a stepped scraper of the correct radial depth and height which I duly collected along with a handy selection of other rings with which to experiment. This ring also gapped massively and, ever the perfectionist, I ploughed on and fitted it together with the best of the compression rings that could be mustered.
Hey, what do you know, we made a super-efficient oil pump! And although the smoke was horrendous at least it was dense enough to cover my shame when stopped at traffic lights. After 20 or so miles the situation hadn’t improved, the plug quickly oiled up and after a £10 taxi home I collected the bike in the van.
The bike felt good with the lighter piston and the loss of 0.5 : 1 compression from the Australian item was not immediately noticeable. In an ideal world we would use a 7.5 : 1 as per the one in the photo at the top of this post. If anyone has a good ‘un available, please let me know. These are a little heavier than the 7 : 1 but should work well on today’s fuel. Interestingly, the Sunbeam pistons are stamped with a “7” or “7 5” on the inside of the crown to identify the type.
Failing that, we may have to get some new pistons manufactured if we can find a cost-effective supplier. So, watch this space or contact me to place your order….