I couldn’t resist sharing this video of the new forged Sunbeam Longstroke piston undergoing its first trial run. Thanks go to Archie Beggs and Kenny Crawford who kindly put the piston into an engine they are preparing for a friend and then demonstrated it on their high-precision, custom-built Sunbeam engine test rig. The eagle-eyed may notice the lack of a cooling fan but Kenny had to fit it back in the bathroom as the condensation was causing problems.
Following on from the successful production of pistons for OHV 500cc Sunbeam models Chris Odling and I thought it would be useful to produce some high-quality forged pistons suitable for the Model 6 side valve long-stroke. The Model 6 was produced from 1922 until 1939 becoming known as the Lion after takeover by ICI. With its 77mm x 105.5mm bore and stroke this was the last side valve powered motorcycle to win a Senior TT.
There were of course changes along the way and the piston produced is nominally compatible with models produced from 1925 until 1929. Before 1925 a 5/8″ gudgeon pin was used rather than the later 7/8″ type and the change to a scavenge pump from total loss oiling in 1930 means a different piston ring configuration is required. It is possible that these pistons could be adapted to suit.
I have limited availability of pistons in 77.0mm +0.000″ and +0.040″ sizes. An all-up weight of 434g has been achieved for the 77.0mm which is within 2g of the target weight that we set out to achieve.
Once we have given the new items a good look-over I’ll be contacting those who have reserved pistons already and adding them to the Sunbeamland shop.
It’s good to see our friend Carsten Conrad isn’t shy about using his bike as the maker intended. The eagle-eyed may notice the modern rubber which he prefers having covered around 1500 miles last year, often riding in company with more modern machinery. Averaging 54km/h over a 220km ride on back roads and through small towns hints at some serious throttle abuse.
Perhaps not surprisingly he is now looking for more performance from the motor: “The bike runs good but not yet enough”. With a new piston, genuine TT90 head fitted with large valves and his own valve guides, the target is now 30hp. I suspect that it won’t end there…
I recently donated my own spare Amac 10MDY to a worthy cause but if anybody has one that they could pass on to replace Carsten’s Amal 27/013, please drop us a line to and we’ll put you in touch.
I’m pleased to report that a rather pretty box containing a small batch of low-compression Sunbeam pistons arrived this morning. Like the 7.5 :1 CR Model 90 versions produced earlier, these are a lightweight forged item but will give around 6.5 : 1 CR depending on the head and barrel combination in use. Targeted at the Model 9 it is suitable for late 20’s early 30’s 500cc OHV bikes and we have limited availability of both +0.020″ and +0.040″ over-sizes.
Our research showed that the weight of Model 9 pistons varies surprisingly depending on both year and manufacture. After collecting and checking pistons from Marston, Heplex and Specialloid a target weight of 400g was achieved for the new item – including rings, pin and clips.
Once we have given the new arrivals a good look-over we’ll be making contact with those who have already reserved their pistons. So be quick if you don’t want to miss out!
I’m pleased to say that we now have the recently manufactured high-quality, lightweight forged 7.5:1 pistons available on the Sunbeamland Shop in +0.020″ and +0.040″ oversizes. Modern materials and CNC machining mean that all-up weight is within a few grammes of the original Marston’s item and I’m enjoying the experience of running one in on my own 1930 Model 90. They should fit any of the late 20’s early 30’s 500cc OHV bikes and we hope to be following up with production of a small batch of 6.5:1 pistons aimed more at the Model 9.
Forged piston for Sunbeam Model 9 and Model 90 7.5:1 compression ratio
This little beauty arrived today and I couldn’t wait to put a post up on the blog here at Sunbeamland. It is the first from the box of a batch of forged Sunbeam Model 90 pistons. It is modelled on an original Sunbeam 7.5 : 1 piston and, unlike some alternatives that are available, weight has been controlled so that the combined weight of piston, pin and rings closely matches the original.
Next job is to bore a spare cylinder to suit either of the +0.020 or +0.040 sizes we have produced and get some testing done before winter sets in.
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….