In the shop

500 twin pistons

While I'm about a particular part of the bike like girders or front brake I am well up on it, but years later the detail starts fading away. Anyway I am currently on 500 twin (63mm) pistons and have gathered a bit of gen so will try and keep records this time. 

My '51 Tiger 100 needed a rebore. I put it together in a hurry years ago as I had too many projects and knew the bores were worn but thought it more important to get it running. I had seen loads of listings for new 500 pre unit pistons in all the oversizes and have also now found a good engineer locally (Paul Reeve at PR Motorcycle Engineers, Tasburgh, Norfolk. Highly recommended).  Fortunately the barrels were still on std bore and after measuring, Paul confirmed that a +20 o/s would do. I still had the original pistons which are 7.6:1. I started looking harder at the options for new pistons and asked around. The consensus was: assuming original Hepolites are not available, then LF Harris are OK. However they only do the low compression 7:1 (5T & TR5). I contacted FW Thornton (Long-standing piston suppliers) and they said they could supply a pair of 7.7:1. These come from an Australian company JP pistons.  A bit more expensive at £190 but seemed a good idea. However, further research reveals they are unpopular (or worse according to some sources) Problems revolved around the extra weight (only good for banging in tent pegs was one comment), the quality of the rings, and too shallow a groove for the pin circlips resulting in them coming out under stress (a bad thing). I really wanted the correct C/R so bought a pair, thinking I could fit LF Harris rings, use loctite on the pin circlips, and maybe even take some material out of the inside to reduce the weight. On receiving them I was surprised at the weight difference. Each bare piston weighed 25g more than my originals. On top of that, the pins were heavier (by 12g each) resulting in an all up weight increase of 74g. This was bound to affect crankshaft balance adversely. Then the final straw. The gudgeon pin was too big for the small end bush. I sent them back and to their credit Thorntons refunded me without a murmur even though I was outside the refund deadline by quite a bit.

So, it's getting complicated. It seems that original E 1908 Tiger 100 pistons are not only higher C/R but lightened (making the JP comparison a little unfair). Even the gudgeon pin is 6g lighter than an original 5T piston pin, being machined out to a shallow cone inside. I started checking piston part numbers, set out below:

E 1563 (7:1) listed for 5T, 1939 -1956

E 1716 (C/R?) listed for T100, 1939/40

E 1908 (7.6:1) listed for T100, 1946 - 1956

E 3098 (8:1) listed as option for T100 1954, then 1956 - 1959.

Original Hepolites used the Triumph E numbers (above). However, at some stage a long time ago - not sure when, Hepolite became Heplex. They used generic, or at least their own numbering which seems to be SW prefixed. In addition, it seems this second generation of 'genuine' pistons was no longer lightened for Tiger 100: I have now managed to find a pair of +20 Heplex 7:1s, as well as a pair of +40 7.6:1s (Triumph said don't bore out alloy barrels past +20 o/s so I guess these are for iron barrel T100s). Interestingly, the Heplex Tiger 100 (SW 11563) pistons weigh in at around 200g each, 25g heavier than the E 1908s, and the same weight as their low comp. equivalents.

However, I still think the best option is to try and find original pistons. Apart from anything else they seem to have been made of a clever, low expansion silicone alloy. This is important because it governs the clearances within the bore at the machining stage. So currently I am planning to fit the low comp Heplex pistons but am delaying the build for the time being. You never know, I might find a pair of +20 E 1908s . . .