Today I stripped the engine of my '720' M20, built around 12 years ago...
During that time it's done a lot of work, both local and longer runs including dual carriageways, motorways, off road,Normandy tours etc. etc.
Originally it was put together with a high content of NOS parts...
After a couple of years, following some Ex.valve problems I fitted a modified exhaust valve from an Iveco turbo diesel which had a hard chrome plated stem and Nimonic 80 head to go along with the hardened exhaust valve seat I fitted at the outset.
The piston used was a forged 90mm J E piston...Crank,M21...NOS BSA big end.
I don't thrash my bikes particularly but like to use what they have..Anyone who has ridden with me will understand
This is what I found...
Some wear in the inlet valve guide and wear on the valve stem, sufficient to warrant replacement of both parts.It was a NOS BSA valve...Valve seat and face good..Inlet valve spring within tolerances..
No appreciable wear on the exhaust valve but the guide was worn. The valve face and seat are virtually unmarked...
The exhaust valve spring is outside the laid down tolerances.
One broken compression ring (top)...Virtually no wear and little marking on the piston thrust faces...The bore has some wear but I haven't measured that yet to determine the extent of it...
Small end bush and gudgeon pin worn...Both will require replacement...
Inlet tappet head worn. Exhaust tappet head less worn but I will replace both..(Both NOS originally).
Slight wear on the apex of both (NOS) cams...This could easily be removed with a light application of an oil stone.
No appreciable wear on the tappet faces or of the tappet guides and stems..
No appreciable wear of the (NOS) oil pump drive shaft, cam spindles, intermediate gear spindle or crank pinion shoulder.
Intermediate gear bush slightly worn, sufficient to warrant replacement.
No wear on drive side main bearing spacer. Oil pump turning freely in both directions..No discernible wear in big end..This will be stripped for a closer examination...
All main bearings in good condition...
All in all not too bad for an engine that has been stretched by 220cc beyond its original capacity and has been used reasonably hard over a lot of miles...Ian
email (option): ian@wright52.plus.com
I still have some NOS valve guides so will use those for replacements...
You are right, new guides are parallel and I have also found some that are slightly smaller on the O.D. Saying that though, I've had new ones that are nearer the maximum size as well so there is some variation...
I don't have anymore NOS inlet valves so will use a G&S Valves plasma nitrided valve for the inlet...That should be better than the original...
The exhaust valve doesn't require replacement...
I have NOS Terrys valve springs so will fit those as well....
The engine was actually running perfectly well, I stripped it mainly to refinish it cosmetically so that it matches the rest of the bike. That needed restoring as the paintwork wasn't really protecting the tinware anymore in many places, forks were worn, rear stand needed repair etc.....
I must try to resist the temptation to ride on the beach (in the salt) every time I go to Normandy..
...Ian
email (option): ian@wright52.plus.com
Hi Henk..I've run with both set ups at different times...I use C15 inner springs when I fit them....
This engine was the first 720 I built and it has always had standard springs...
I think if you're not the type that likes to rev through the gears that set up works OK...particularly so if you have a full weight crankshaft..
I'm considering lightening the flywheels slightly this time to improve 'pick up' and acceleration...Nothing radical but I will probably fit the inner springs if I do that...
The 'brown' bike that I sold to John Allison did have the crank lightened slightly and I ran the inner springs in that one...Ian
email (option): ian@wright52.plus.com
Glad to hear it's all sorted John...I hope you have recovered from the 'knock' as well...
The engine should run for a few more years yet before it needs much work...Ian
email (option): ian@wright52.plus.com
I'm very pleased to find this report on 720cc engine longevity. I have just got my "Ian Wright" 720 Piston Kit installed after a lengthy delay; machine work required to bore cylinder and installing a hardened exhaust valve seat (original seat found to be cracked) on the well-used unlined barrel I finally located.
I'm happy with the engine performance and very much enjoying the extra pulling power the increased displacement provides.
Thanks Ian!
email (option): teladelujo@msn.com
Meant to include this in the previous post....
email (option): teladelujo@msn.com