My Father always told me I shouldn't mess about with things that I don't understand. I have spent the last 55 years or so ignoring that advice but now, just short of my 60th birthday, his advice has a whole new meaning and it's all thanks to my Triumph.
Before I get in to this I would state that I am no expert, the following is my take on matters, other people's opinions may well be different. Any discussion around the issues on forums etc can get quite lively with different opinions being expressed forcefully. It's fine to diasagree, this is just my take on things.
I have always messed about with Japanese bikes produced from the 70's onwards and there are certain things done in certain ways, things that the Japanese seemed to understand better than we Brits and things that allowed them to make better bikes and kill off our bike industry.
One such thing became rather apparent when I was trying to find a new cylinder barrel to replace my badly corroded one. I wanted to retain my virtually new high compression pistons, which at plus 30 thousandths were an unusual size , the engine had previously been rebored, it did not appear to have done much running at all since. This is where it all started getting a bit weird, strange and slightly stressful. With me being used to the Jap stuff tolerances and machining sizes are one thing but when you look at 60 year old British engineering they become quite another. You see back then they didn't have the sort of smart alloys that our Nippon friends were brewing up back in the 1970's. The stuff Triumph used had this dirty habit of expanding and contracting much more with temperature changes than the Japanese stuff so different tolerances had to be used to allow the engines to work correctly and efficiently over a wide range of both ambient and operating temperatures. The Japanese with their stable alloys manufactured to much tighter tolerances and didn't have to worry about alloy pistons in iron bores going tight at high temperatures. The fact that the blocks were made of stable alloys as well helped matters tremendously.
Anyway, my problem was the pistons that came with the bike said they were plus 30 thou - half a millimeter is 20 thou - hold that thought for a minute. A standard Triumph 650cc bore is 71mm, rebore sizes go up in 0.5mm or 20 thousandths of an inch steps, so my plus 30 pistons ain't no standard size that anybody can supply. I found a used barrel that said it was plus 30 but when I received it I measured it and found that it was just a tad under 72mm - that would have been a plus 40 overbore size. My original barrels measured exactly 71.5mm so they were over bored 20 thou, not 30. I think what has happened is that whoever built it for racing put in tighter pistons, possibly to quieten down noisy piston slap at cold - piston slap caused by Triumph having to allow extra tolerance to allow for the expansion I went on about earlier. Piston slap is where the bottom of the piston skirt slaps against the sides of the bores, it's an annoying noise which usually goes away as the piston heats up, expands and fills the bore a bit better.
I measured mine as best as I could and found that the gap was about 6-8 thousandths of an inch so I should get away with it. I understand you can fit Teflon bungs that stop slap totally but they wear out so hardly seem worth bothering with. I also understand that the very best pistons are very slightly off round to give better, closer tolerances but they are incredibly expensive if you can find them.
So where I am now is that I intend to use the new 72mm barrel that I received, the bores are in excellent condition, a very light hone was all that was needed, I shall use my original pistons and fit plus 40 rings, of which I can file the ends down if needed. If it is too rattly and it gets on my nerves I will buy some plus 40 pistons, they will have to be high compression ones to allow me to continue running on methanol. I think I am right in saying that Triumph used a variety of compression ratios over the years ranging from a lowly 7 to 1 right up to 12 to 1 - methanol requires something towards the higher end of the scale, the more the merrier.
It has taken a fair bit of messing about, a lot of reading and a lot of talking to better informed people to get this far. I have ordered parts that are no good to me, that's no big deal, they will always sell on without problem, even my knackered barrel will be of use to somebody that will get it rebored to plus 60. I now feel I have a much better understanding of the engineering of these bikes, I also understand why the Japanese bikes with their incredible power and reliability became so popular. Don't get me wrong, I'm not knocking the British stuff, in it's day it was brilliant but the industry was complacent and never saw the Japanese threat until it was far too late. Often British bikes were built on worn out machines and the quality was quite variable, get a good one and they are an absolute delight, a bad one not so much. The ones that are about now tend to have been updated, modern ignition solutions are available, there's no need to run a magneto any more unless you want to. New cylinder barrels, pistons, rings, bearings etc are all available made from modern materials with modern levels of accuracy. My bike is all about originality, it's a nostalgia trip for the dedicated non follower of fashion, a passion project for the masochistic mechanic.
Opinions remain divided as to whether this is 24 carat or fools gold, I know what camp I am in, very much the former rather than the latter. I have seen some comments to the contrary, which is fine, hopefully those opinions may change when they see and hear the finished bike thundering down the quarter mile.
All this means I won't be able to put it all back together over Christmas as the parts won't arrive in time, so I shall spend the week drinking heavily instead. I am in much the same boat with my little Maserati 50cc racer, it's most annoying. My Mrs said something about smashing my stupid little face in if I so much as mentioned the workshop on Christmas day anyway so it's probably best to have a break for a couple of days at least.
The original barrel with the original plus 20 ring, note the very small gap on the ends of the ring - this is just what you want to see. If there was no gap the rings could bind with the engine fully hot, too big a gap and they just don't seal properly.
The same ring in the new barrel, note the big gap - far too big for the engine to run properly, if at all.
This dimension should have been 71.5mm, which would have been a plus 20 over bore but it's nearer 72mm, which is a plus 40. The pistons I have are plus 30's but they should be ok fitted with plus 40 rings.