Discussion in 'Norton Commando Motorcycles (Classic)' started by auldblue, Oct 17, 2018.
Recent engine strip reveals damaged con rods , what is the reason for the wear? Is it the crank?
Its from the crank or a rogue piece of something but could be caused by the bores being out of kilter or the piston and forcing the rod off to one side. Would have to have a good look over all the parts for other witness marks and measurements to see why it's happened. Could also be a piece of something got trapped for a short period.
One side only? One rod only? More information please.
One of my racebikes had stock rods with grooves worn much deeper than the rod in the photo above. I attributed it to crank flex. It could have been from misalignment but I couldn't find anything that far out of wack.
I wonder if your crank has been reground and removed the .090" Rad allowing the rod traverse to close to the edge?
The fillet radius does not (should not) constrain rod movement longitudinally along the rod journal. The bearing shells are considerably narrower than the rod big ends.
Correct, always amazed me how much space seems to be wasted on the edge of the shells and then there is the chamfer on the side of the shell on the bearing material reducing the bearing surface area even more.
Is there a corresponding mark on the crank. Could be something trapped as others have said, could be a ding in the crank sticking out, but if it is all the way around or nearly so, I would suspect alignment. The answer to the earlier question - it is one rod or both would help determine where to look next. Also, it appears to be on the inside of the rod (flywheel side) - is that true?
There is damage to the other rod as well but the rods are on their way to Norman White to have a look at with some other parts. The crank is std and was polished by SRM along with the rods. They supplied shells at the time but when I fitted them the rod bolts would not tighten properly, as it turned out they were for a reground crank and when std shells were offered up they fitted with no problems. The crank end float was 25 thou on fittment on reflection I should have pulled the inners and shimmed it down to 10'thou but I did not do this which makes JS theory of crank flex maybe the answer as SRM checked the rods and said they were OK.
Regardless of crank shimming, there's plenty of slack between the small end of the rod and the piston boss to allow the rod to drift on the big end journal from side to side. I seem to recall 0.025" crank end float as an acceptable limit.
I raced without a tac and over revved to the point that the .050" mockup clearance between piston and head wasn't enough. Auldblue - if this is a race bike or a highly revved cafe bike then yes it might be crank flex causing the problem but if its a low/med rev streeter then I'm not so sure.
Just thinking aloud, disregarding the radius and debris theories , as you say both rods are marked, and appear to be making contact with the crank , acknowledging that crankshaft end float is an unknown quantity, It may be worth checking your rod to piston clearance around the small end eye, could the cause of problem be at the top of the con rod and the marks around the big end eye be a symptom , what pistons are you using? has the bike been rebored/overbored.
If the rods are straight maybe the caps got mis matched by PO ? Just a shot in the dark.
Misalignment of the rods on the small ends was suggested and on reflection there was a slight rattle from the engine on its last trip when really hot(ran for a good few miles down on oil ,service station closed for refurb)and it was suggested that it may have been coming from the small ends. The crank journal radius were measured at .08 on three but the timing side inner measured .1 , crank journals in good shape standard clearance. This engine was a mix n' match job with parts collected from Evilbay and new parts from AN.
Pistons are 73.5 , the engine was last run on a dyno in May 18 ,revved up to 7000rpm and there was no strange noises, well non that I heard anyway.
Rod caps seemed to match marks on rods, but again bought off eBay??
73.5mm pistons suggest a couple of rebores. I would look at the individual bore alignments with the crankshaft. Somewhere along the lines the someone described the assembly of Commando engine as that of a Dagwood Sandwich.
I don't believe our friends across the pond will even know who Dagwood is.
Agent google, supplied the the intel on Dagwood
A long shot, the iron portion missing will collect on the magnetic plug, the alloy portion missing form the rod should have been caught by the sump strainer, if nothing found it suggests that the marks could have been there when installed. If they have passed through the strainer then look in return oilways.
I will return to the crank later, cheers for the replies but a recon it's time for a Dagwood sandwich!
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