Forum

[Ameya]

Hello all,
I want to perform a top end overhaul on my 1989 Indian Royal Enfield 350cc. I have brand new +40 piston that I want to install. Can anyone help me with the corresponding bore diameter and tolerance I should ask the machine shop to maintain.
Thank you
Regards,
Ameya

[Mark M]

Hi, who made the piston? The reason I ask is that the better quality the piston the tighter clearance you can run. Otherwise, the old rule of thumb is 3 thou per inch of bore so on a 70mm bore that's about 8 thou. However, it's late and I've just got back from a good evening blast on the Interceptor so perhaps I should check the manual in the morning!

[Adrian]

8 thou seems a lot Mark, ISTRC our hosts only recommended 6 thou for one of their Accralite forged 87mm pistons.

A.

[Mark M]

Adrian, that's why I asked where he got the piston from so the figure could be narrowed down a bit! Since the poster is probably one of those folk who ask a question (or a lot of questions,) and never bother to post back, I will suggest this goes in the 'what oil' department along with all the others! Now, I really need to know how long is a piece of string for use in a....

REgards, Mark

[Sticksy]

For that particular application, 2 1/2" is about right - that is the factory figure if using Redditch made string.

[Mark M]

Ha ha! Piston to bore clearance is subject to so many factors, the rule of thumb I gave was often applied to pre-War designs, I'm guessing an Indian spec 350 will be fairly basic. Modern low expansion pistons like the JEs I'm about to install in my S2 Interceptor run much tighter. Anyway, we'll probably never know!

REgards, Mark

[Alan R]

Hi Guys--------- I wonder if our hosts could be persuaded to start an FAQ ( Frequently asked questions ) thread as a "Sticky" ??.... Or we could have a downloadable PDF with a Q's and A's format ---------- or maybe a printable version that you could clip together in a binder ??------------- or a Professionally published version with a snazzy title such as}---- "Workshop Manual" ?? -------------- Oh dear, full circle methinks !!.........AMEYA}---- Assuming your piston came supplied in an appropriate box then there should be a piece of paper enclosed giving all the details that your machinist will need to know........ If however this "Brand new" piston was sourced at a Jumble, or from a friend etc.---etc. then you should proceed more cautiously.....Factors such as}----- 2-stroke or 4-stroke ??------Air cooled or liquid cooled ??-----Intended compression ratio....Piston crown design can have an effect...----> With that last one I'm referring to our OE pistons and their Kamikaze-style decapitation tendencies that have been well documented down the years.....Make sure that the machinist uses a good hone to obtain final diameter........The cross-hatching helps with bedding -in and oil retention..Also, what ring gap will you be using ??

[enfield_trials]

Hi Alan R , I agree to your point of view. if we have FAQ or basic troubleshooting document. the same questions are repeated after sometime.
i was thinking two weeks ago , going to previous forum questions archive build document. Regards

[papasmurf]

What is wrong with just looking in the workshop manual? (Ducks and runs.)

[Presto]

This is a figure the piston manufacturer always provides. The piston should be given to the engineer and he should be instructed to allow a 4.5 – 5 thou clearance if it’s a 'straightforward' replacement piston for a 500 Bullet. [5.5-6 thou for a forged piston].