Piston clearance...

Fast Eddie · Dec 31, 2013

Gents,

My 850 was on std bore. It didn't burn oil and it is a very quiet engine, no audible piston slap etc.

But when I stripped it, due to the fact that the PO had honed the bore, it was actually running with 8 thou clearance!

Clearly this is way out of spec (and I'm going to +20 pistons from JS) but it is interesting to me to note how well it ran whilst being so far far out of spec. I wonder how many motors are needlessly re-bored?!

concours · Dec 31, 2013

Fast Eddie said:
Gents,

My 850 was on std bore. It didn't burn oil and it is a very quiet engine, no audible piston slap etc.

But when I stripped it, due to the fact that the PO had honed the bore, it was actually running with 8 thou clearance!

Clearly this is way out of spec (and I'm going to +20 pistons from JS) but it is interesting to me to note how well it ran whilst being so far far out of spec. I wonder how many motors are needlessly re-bored?!

Ok, I'll bite.. Define "needlessly". What were the compression readings? Piston ring end gaps? Lawnmower engines run very well on low compression, (don't produce much power). How is the bike ridden? Like a bank robber? Or sunday morning parade duty?

olChris · Dec 31, 2013

Fast Eddie said:
Gents,

My 850 was on std bore. It didn't burn oil and it is a very quiet engine, no audible piston slap etc.!

If its not broken don't fix it...

Fast Eddie · Dec 31, 2013

concours said:
Fast Eddie said:

Gents,

My 850 was on std bore. It didn't burn oil and it is a very quiet engine, no audible piston slap etc.

But when I stripped it, due to the fact that the PO had honed the bore, it was actually running with 8 thou clearance!

Clearly this is way out of spec (and I'm going to +20 pistons from JS) but it is interesting to me to note how well it ran whilst being so far far out of spec. I wonder how many motors are needlessly re-bored?!

Click to expand...

Ok, I'll bite.. Define "needlessly". What were the compression readings? Piston ring end gaps? Lawnmower engines run very well on low compression, (don't produce much power). How is the bike ridden? Like a bank robber? Or sunday morning parade duty?

A fair point...
I mean 'needless' in this context as there was no evidence of piston slap, and no (and I mean zero) evidence of blow by on the pistons. I am not sure, but I think the PO put in oversize rings and gapped them appropriately.
However, in agreement with you, it depends on ones own 'standards' and ones own riding style... I am a fussy bugger, and I want a bike I can thrash, so I am re-boring and fitting JS pistons, rods, cams etc.
But for the type of classic rider who seldom goes above 5,000rpm and only does a couple of thousand miles a year... He wouldn't notice any ill effect at all.

Fast Eddie · Dec 31, 2013

olChris said:
Fast Eddie said:

Gents,

My 850 was on std bore. It didn't burn oil and it is a very quiet engine, no audible piston slap etc.!

Click to expand...

If its not broken don't fix it...

Compression ratio was too low for my wishes Chris, so new hi comp pistons were required anyhow to 'fix' that issue, so a rebore was an obvious job to do in this case.

concours · Dec 31, 2013

My 850 started and ran fine, no smoke, with the bores being scored and rust pitted. Looooow compression though

Hobot · Dec 31, 2013

ugh but very informative satisfying ugly bores on what a slip shod mechanic like me can expect to get away with Concours. No one making cast iron replacements I know of so each rebore is that much less Commando remaining. Thanks goodness Norton made more than enough to last us a while. Wes and I go round and round on Nth degree details on his and my C'do's cheap as we can good enough finishes and half the time he's right and the other half I am. Your views and reports support my view point on that particular surface issue in Trixie that don't smoke no more but still weeps on 2S powerband. Best wishes on least blow by a long time.

Fast Eddie · Jan 1, 2014

hobot said:
ugh but very informative satisfying ugly bores on what a slip shod mechanic like me can expect to get away with Concours. No one making cast iron replacements I know of so each rebore is that much less Commando remaining. Thanks goodness Norton made more than enough to last us a while. Wes and I go round and round on Nth degree details on his and my C'do's cheap as we can good enough finishes and half the time he's right and the other half I am. Your views and reports support my view point on that particular surface issue in Trixie that don't smoke no more but still weeps on 2S powerband. Best wishes on least blow by a long time.

Its easy to re-line stock barrels.
And there are new iron barrels around, Andover, RGM, Hemming and Norvil all stock them. 750, 850 or even 850 style with 750 bore.
Any colour you like, as long as its black.
The choice is yours sir!

olChris · Jan 1, 2014

Never forget the air cleaner Those pic show to me that sand, dust, grit and water has been ingested over time.. Air cleaners are there for a purpose and the standard arrangemet for Commandos is piss poor for any dry, dusty conditions.. OK for snow and rain and damp, but not for me.....

concours · Jan 1, 2014

olChris said:
Never forget the air cleaner Those pic show to me that sand, dust, grit and water has been ingested over time.. Air cleaners are there for a purpose and the standard arrangemet for Commandos is piss poor for any dry, dusty conditions.. OK for snow and rain and damp, but not for me.....

+1 the flexible tin box can't seal well without lots of modification.

acotrel · Jan 1, 2014

I think there is such a thing as ring flutter. The piston speed in a long stroke commando is close to the theoretical maximum, and bore clearance might have a bit more effect than usual ?

X-file · Jan 1, 2014

Apart from good ring groove clearance,the best thing you can do to avoid ring flutter is to use 30% bigger gap on the 2nd ring compared to the top ring gap.It also improves oil control.

I wouldn't re-bore a cylinder until it showed about 0.007" wear/taper.

kommando · Jan 1, 2014

Assuming the 8 thou clearance was the full clearance then that was only 3 thou wear over the original clearance of 5 thou, no need for a rebore.

Fast Eddie · Jan 1, 2014

kommando said:
Assuming the 8 thou clearance was the full clearance then that was only 3 thou wear over the original clearance of 5 thou, no need for a rebore.

Correct, 8 thou was the total.
What is the upper limit for wear in these engines?

Al-otment · Jan 1, 2014

Fast Eddie said:
kommando said:

Assuming the 8 thou clearance was the full clearance then that was only 3 thou wear over the original clearance of 5 thou, no need for a rebore.

Click to expand...

Correct, 8 thou was the total.
What is the upper limit for wear in these engines?

0.005" according to the Mk3 manual. So you're over half way - I'd rebore whilst it's apart.

X-file · Jan 1, 2014

I totally agree with kommando,0.003" is nothing and it's probably just starting to run good at that.
Back in the early '80s,I rebuilt a friend's T140 that had 0.007" bore taper/wear.It ran fine with a hone and a re-ring,and went better than ever before.It hasn't been pulled apart since,and still runs fine (apart from a small recent oil-leak).

Hobot · Jan 1, 2014

If just runing around why not try bit bigger rings gapped better with 2nd a bit bigger gap as mentioned and see what happens, if that don't do it then rebore then reseelve after that as Fast Ed mentioned too. Good breather valve may give bores some long service too. If ya run your C'do to valve fluter rpms, w/o spending on every thing else, ugh.

Piston clearance...

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