Pistons and compression ratios

I know nothing about Norton singles, but i do run a high compression hot cammed single. Its great for fast "A" roads and the Isle of Man. Its also a bit of a grouch to kick start without a valve lifter and I would happily swap it for a soft single for those club runs in the back lanes. Sounds like you already have a good compromise.

Hi Andy, Ducati 250 Mark3 NC with Mach1 spec motor ported ,40 mm inlet, F1 cam best power 9500/10000rpm. Ton up moped!.

The shop I work in does a lot of work on old Jags and Alfas with a very similar combustion chamber to the Inter. Our customers' experience seems to put 9.5:1 at about the max on pump gas. Get some 110 octane race gas and all bets are off.

In the 1930s, 7:1 was considered pretty high compression. There is a lot more to power output than compression ratio. An Inter on 7:1 should go rather faster than an ES2. 9.5:1 is a bit high for present day petrol. If it were up to me, I would stick to the 7:1 and accept a better-mannered engine and avoid the possibilities of pinking and detonation,known to be detrimental to for the engine's health.

Ignoring the ethanol distraction (several fuels, in the 30's, contained this), modern premium unleaded petrol is of a higher octane rating than normal pump fuel obtainable before the mid 50's. Probably, anything over 9.5/1 would be excessive, but, possibly with slightly retarded ignition, there should be no problems. After all, I don't suppose you are going to ride all day at race speed.

Unless you are sure of a perfect bottom end and are prepared to chance a blow up with loss of case/barrel i would stop at 8 to 1. Is it a 350 or 500?. In any case its a very old gentleman and should be taking it easy in retirement! ,just like me.(not).

Previously andy_marks wrote:

Its a 500 and the bottom end is good (was all new when I built the engine 2 years ago). I may well leave it as it is, but am just interested in what other people with inters and manxes run and what I could get out of mine reasonably safely.

Andy I run a'37 Plunger manx ( Long stroke magnesium), no kickstart. 9.5:1. Its been in this running condition for many miles, with cracksin mudguards, tanks,seat pan(yes it has a full length seat) and frame (sorted witha replacement at one point) displaying itshard use. Itstarts on rollers and is fine once warmed forstarts on general runs as long as you pay attention to where you park. Butwhen you ride it.... itslike nothing else, smooth, fast,a bit wobbly compared to a rigid but stunning performance forsuch an old one.... Ridden asintended,kept like they use to be; fettle before failure, no more.Always use best quality fuel available and keeep the oilclean, the restistestement to your preparation.

Cheers Jon

Hi Andy

I run my !936 350 Inter on the road in full touring trim. It has an iron head anda cr of just over 8:1and I usestandard pump fuel. I havealso added a cartridge oil filterwhich is fitted beneath the toolbox and use standard 20/50 oil.I have been running the bike in this guise since 1986. An earlier thread was talking about using a TT carb on the road which lacks a tickover facility. I find that with the throttle friction screw set so the throttle doesn't automatically close I can normally set a reasonably steady tickover when I stop. For a ridgid bike I find her quite comfortable andgood for long distance riding apart from the oil it leaks, but it stops me worrying about the back end of the bike rusting! As a comparison I also have a WD 16h and the difference is like chalk and cheese, but it is great for thrashing over shorter distances.

Nick