Is any UK based member of the forum running a Triumph twin?.. (not a Hinkley model)...

If so what compression ratio are you running and have you experienced any problems with pre detonation (pinking)....?

The Triumph has a very poor combustion chamber shape and can suffer with the modern fuels...I'm contemplating 9.5-1 for my Triumph big bore kit but am a bit concerned about problems with pinking...
I can get 8.5-1 pistons for the kit but they are not 'off the shelf' items and will lead to a delay in supply.... ....Ian

email (option): ian@wright52.plus.com

Ian, not sure if this is any help to you. I think you're looking at earlier twins than mine (i.e. 650cc bikes).

But I've now got three T140s. One is actually a 750cc TSX running BIng carburettors. The other two are running Amal 930s.

The two Amal bikes do pink a little, but you have to be selective with fuel. I avoid supermarket stuff, but I'm not sure if the supplier is the same, or if I just don't like buying from ASDA and Tescos, et al. But I have at times definitely noticed extra pinking with supermarket fuels (mostly in hot weather - not that I can remember what that feels like). Not so bad in the damp and cold, of course.

The TSX doesn't pink. Recently I rode that over 250miles through rain storms and through Derbyshire hill. No pinking problems.

But T140s are running either 7.9:1 compression ratios, or more likely just 7.4:1 using the pistons generally available on the market. The 650cc bikes I've ridden and tested are much the same. I think the ignition needs to be retarded ever so slightly and you have to watch it more closely with the gears.

Where possible, I used Super Unleaded or similar.

That probably hasn't helped much, except that if T140 pink on those (relatively) low compression ratios, the 650s will probably have a harder time. That said, T140s have got odd cams that stifle breathing and spoil the gas flow. All the bikes are running Boyers, and I'm advised that the advance curve isn't great on those ignition systems. That might be part of the problem. Lucas Ritas are supposed to be better. Can't speak for Pazons.

Finally, one of my T140s has different (from standard) cam timing. I've forgotten the numbers (written down somewhere), but a T140 "expert" advised me to try them, and I dialled them in. The bike definitely runs better, faster and maybe a little cooler. But there is still a tendency towards pinking, albeit much less so.

Any use to you?

email (option): dannydefazio@sumpmagazine.com

My 1969 650cc Trophy (now sadly gone) came to me with 7.5-1 pistons as the previous owner was going to hang a chair on it. I upgraded it with 8.5-1 pistons off the shelve from Brit Bits. There must have been an increase in performance but I never really noticed it, and it never pinked at all. I fitted Boyer ignition. I once rode it on the Dorchester to Bere Regis duel carriageway at 90mph the whole way. I now tend to rather look at the view. Ron

email (option): ronpier@talk21.com

Thanks for the info so far...The kit I have bought has an alloy Nikasil plated cylinder so may well run cooler than the standard set up. It boosts capacity to 750cc whilst retaining the original 650 cylinder head...

As the capacity boost is all in the bore and the stroke remains unaltered if the pistons are increased from the standard 650 bore of 71mm to a T140s bore of 76mm then the capacity would actually be 744cc..

The pistons supplied in both 8.5 or 9.5-1 ratios are therefore of a 'squish' design, the squish being the difference in diameter between the two bore sizes quoted (5mm or 2.5mm all around the bore). (Reputedly) These are less inclined towards pre ignition than the standard type...

If my memory of previous Triumphs is correct compression ratio can be lowered by one atmosphere using a thicker than standard head gasket...However, that would negate the benefits of the squish area...

I'm fairly confident that at 8.5-1 it would be 'trouble free motoring' as they say...but I have my doubts about 9.5-1...

I can run that compression ratio in a BSA single but that hasn't got the Triumphs overly deep combustion chambers and poor valve geometry...and I know T140s can have problems....

Any further relevant comments from other Triumph owners appreciated...Ian

email (option): ian@wright52.plus.com

Ian, there's another issue with the Nicasil kits worth checking. I don't know what your one is, but the Gillardoni kits (if that's the right spelling), were original supplied with the wrong threads (too fine as I understand it). Later kits have the coarse threads. I've got one of those. Never fitted it. But the Gillardoni alloy kits stripped the threads quite happily.

I suspect you've got a different set-up (can't remember if we talked about that on the phone recently, but I've got an idea you might have mentioned Morgo). Anyway, it's something to think about.

Also, isn't your head (no, not that head) 9-stud and not 10? If so, how does that factor into it?

Lastly, funny how we talk about alloy heads or barrels when pretty much all metals are alloys. We ought to be saying aluminium alloys or steel alloys. Pedantic thoughts, of course. But pedantry isn't exactly unknown on this site, is it (and long may it continue)?

email (option): dannydefazio@sumpmagazine.com

Hi Danny...It's an American kit made by MAP Cycles...Available to suit either 9 or 10 stud heads....I stuck with the 9 stud layout as although I'm going to build a 'special' I'd like my 1970 T120 to retain some of the 'keynote' features for that year...

The cylinder is an aluminium forging which is then fully machined before the bore is plated...fairly fancy stuff in comparison to the standard item and as a bonus it's 7lbs lighter than an original barrel...
As one of the targets of this project is to seriously reduce the Bonnies weight (although already fairly light) this will certainly help me in achieving that goal....

Your right about the 'alloys' terminology...slack use of the language and something I usually try to avoid...Innit?...Ian

email (option): ian@wright52.plus.com

Back in the 60s I ran two Bonnevilles, a 65 and a 67. Both bog standard and painstakingly maintained. I had no bother (apart from bits falling off!) with the 65 one. The 67 one however melted a hole in one piston on the motorway coming back from Manchester one day.
I blamed the standard points setup which made it impossible to time each cylinder exactly correct. In hindsight I could have modded the points backplate, but I was only young at the time.
My point is that the ignition timing is critical with these motors, fortunately not a problem these days with modern electronic ignition
Hope this helps, Ian
Regards
Keith H

Hi Kieth..I worked in a Brit bike shop a number of years back and saw a few Triumphs with holes in the pistons...Some through inaccurate set up and some through pre ignition as the petrol octane rating dropped off over time and the compression ratios were too high...

Personally I didn't experience any problems with the T140 I bought new in 1979...Having also owned a Rocket 3 I'm only too aware of the problems that can arise in trying to keep contact breakers set up properly !!.....

I already have a new Pazon electronic ignition in my box of goodies that are destined for the Bonnie...Ian

email (option): ian@wright52.plus.com

Hi Ian
Yes, I agree the petrol didn't help, a new type of Shell came out at the time I had my prob, but I couldn't prove any connection or anything.
I don't think the 750s are as prone to this tho' so you should be OK. Personally I always prefered the 650s, a bit more sporty I thought especialy if you could get Thruxton bits!
P>S> If you think Triumphs have poor valve geometry, I've never had a prob, but Norton heavyweight twins 88, 99, what a nightmare!