Hitchcocks Forum

Has anyone got experience of fitting Mr H's high comp piston in a 350?
I have to have the head off to fix a suspected worn valve guide (smoking after overrun) so I might as well take a look at the piston while I'm at it.
I don't want to spoil the nice "soft" feel of the engine but a touch more grunt wouldn't go amiss.

the piston by itself won’t do that much.



the combination high comp piston, bigger valve’s, ported head, bigger carb will get the most out of it.

alas that a costly affair.



the combination standard piston, bigger valve’s, ported head, bigger carb is a nice one also.

and a bit less costly.



in both cases the engine stays fluffy, with the high comp piston added you’ll get a bit more oomph.



if you din’t need to open the engine, fitting a bigger/beter carb is noticeable also.

preferably a mikuni or MK1 premier.

in the 26mm to 28mm range.

Bigger valves are not needed, over 30bhp is attainable with the standard size[same size as Crusader] valves.

In most 350 the so called hiocmp piston is NOT the ratio stated; lucky if it comes in over 7.5 - 1


But then again the std piston is not as advertised either...


I have found around 0.75 mm has to be removed from the top of the barrel to achieve a combustion chamber capacity of around 48cc to give you the advertised compression ratio.

All very interesting chaps but to return to the question;
Has anyone got EXPERIANCE of fitting Mr H's high comp piston in a 350?
The engine is already mildly ported, free flow exhaust and filter.

Hi Pete, I have one fitted and its given no trouble in three years , the engine is otherwise a standard 58 british engine. A bit more pep but still very tractable. If you fit one just check the height from gudgeon pin to top edge is the same as your old piston, as I have one in my stock which is shorter by about 3mm. unfortuately I dont recall who I purchased it from!

Yes.
Depending on the condition of your existing bore, in all probability you wont notice any difference other than on a drag strip?

Hi Pete as you know I have B31 you may also know it was past rebore size for a B31 piston so I rebored it to take a Triumph T140 piston. His takes it from 350 to 410cc and is higher compression, riding the bike after this showed no real difference in performance,changing the cams for Gold Star clubman cams however made a big difference. In short if your existing piston is OK leave well alone

Not sure I agree, I've fitted the (nominally) 8:1 piston to both the old 1951 350 Bullet and my early 90's Bitsa. The 51 had no other mods and the piston did make a difference especially at low speeds with a more responsive motor at lower speeds.The top speed was much the same and it was largely governed by limited breathing. However reverting back to the standard 6.5:1 piston definitely made a noticeable difference to the response.

Major differences, Simon.


6.5-1 to 8-1


As opposed to 7.25-1 to 8-1


Though as I mentioned earlier, actual compression ratio of the piston is closer to 7.5 - 1


The difference in performance is for the most part not noticed on the Indian Bullet; but each bike is different.