Navigation

You are here: Home / Messages / Commando / 850 Mk 3 stuff (Piston to bore clearance)

850 Mk 3 stuff (Piston to bore clearance)

Up to Commando

850 Mk 3 stuff (Piston to bore clearance)

Posted by mike_steer at September 30. 2017

Looking to have the barrels re-bored +20 this week, fitting NOS pistons and rings from RGM.  What's the recommended piston to bore clearnace for 850's. On R3s and Tridents I have had done the gap was 3 thou.

Also what thread is it in the end of the rocker spindle.  I assume that it's a Whitworth thread?

 

regards

Re: 850 Mk 3 stuff (Piston to bore clearance)

Posted by david_evans at October 01. 2017

0.004" should be good for the bore clearance and ring gap 0.003" per inch of bore. Try the stud from the rear rocker cover to remove the rocker spindles, heat the head first.

Re: 850 Mk 3 stuff (Piston to bore clearance)

Posted by ian_soady at October 01. 2017

I always go for a slightly larger bore than the lower limit as otherwise you can spend the rest of your life running it in.....

The Sunbeam Model 10 I restored last year had been bored very tight and nipped up as soon as it was given any work to do. I had a couple of thou honed out and it was fine with no burning oil or piston slap.

Re: 850 Mk 3 stuff (Piston to bore clearance)

Posted by David Comeau at October 01. 2017

98% of rocker shafts are 26 CEI thread.

Someone is making repop shafts with 5/16BSW or 5/16UNC (hard to tell). Wish I knew who, so I could tell.

bad bad badFrown

Re: 850 Mk 3 stuff (Piston to bore clearance)

Posted by David Comeau at October 01. 2017

delete copy sorry

Re: 850 Mk 3 stuff (Piston to bore clearance)

Posted by K Glassborow at October 02. 2017

Previously David Comeau wrote:

98% of rocker shafts are 26 CEI thread.

Someone is making repop shafts with 5/16BSW or 5/16UNC (hard to tell). Wish I knew who, so I could tell.

bad bad badFrown

Yes I have these in mine....RGM let me into the secret last year when their extractor wouldn't thread in. These are apparently from Les Harris Spares, the former 'new' Triumph builder in the 80's.

Powered by Ploneboard
This is Brio Diazo Plone Theme