crank shaft primary gear problem

[rigsby]

I am in the process of stripping my nc30 race bike engine after suspecting crank shell failure. When removing the primary gear off the end of the crank i noticed scoring on the back of the gear and heavy marking to the point where the crank case has had a bit of material ground away. Iv looked at schematics but i cant tell whether there is supposed to be a thrust washer needed before fitting the gear if so the last owner failed to fit it, if it doesn't then i cant understand as to why it has been catching, the engine was running before strip but making an awful bottom end noise and overheating. Could the crankshaft have moved over somehow due to the possible shell issue.
Any help greatly received.

[lukemillar]

Just had a look through my pics and found this:



But comparing that with the parts fiche and these spacers fit between the sprag and the primary gear - not the case. Only thing I would check is I can't remember off the top of my head if the primary gear has a deeper boss on one side than the other. If so - could it have been fitted backwards?

[Tim400GB]

can you get pictures of the parts/damage?

[rigsby]

HI,

i have took some photos. but cant see how to upload them. i will continue to try!

Surely the primary gear should sit up against the step on the crankshaft. The step i would have thought should be proud of the crankcases so the back of the gear face has clearance when sat up against the step, but the step is slightly inboard of the outer part of the crankcase as you can see in the photos the gear has no clearance hence the scuffing.

Also you can see the scouring on the case and the scoring on the gear face. Its as if the crank shaft isn't central and is slightly further over than it should be. I still haven't split the cases so something maybe more evident.

[lukemillar]

Yep - it would be good to see pics. The gear should sit up against the journal on the crankshaft. Here some pics:





The crank shouldn't have lateral movement if the rods are bolted to it. Post your pics and let's have a look.

[rigsby]

I dont know if the images will work but here they are hopefully






Im beginning to think i may be over reacting it just shocks me to think honda would let the primary gear drag against the crank cases slightly especially on an engine that redlines at 14,000 rpm.

[lukemillar]

It shouldn't be contacting but don't look as bad as I was expecting. If you slide the gear back on - can you see if it is still contacting? How much axial movement is there in the crank?

[rigsby]

There is no lateral movement in the crank but seems overly tight to turn over using a socket on the flywheel side nut. i have more experience with multi cylinder 2 strokes so im not exactly sure how freely the crank should rotate obviously tighter than a 2 stroke with ball bearing mains.
The gear is definitely dragging on the cases as you can see from the photos, surely that shouldn't happen even slightly.

The engine was purchased from a fellow racer and had a refresh then a suspected crank shell failure fairly shortly after rebuild as far as im aware. The guy started to strip it then gave up and bought a replacement engine, i bought it as the top end was fresh also it came with ported heads/lapped valves, quiet gears removed although they weren't on when the engine failed, so i have 2 lots of heads etc. I am rebuilding this one as my current engine also suffered a suspected shell failure and as this one was half stripped decided to rebuild this one as i know the top end is good, getting a bit worried about the condition of the crank now though. Is it viable to have the crank reground all will that wreak havoc when selecting shells?

Thanks for your help

[rigsby]

Forgot to say the cylinder heads are off camshaft idler gears oil pump etc removed so it is just the crank rotating and the pistons moving in the bore, the engine has definitely been apart before as i can tell from the gasket sealent used, maybe it was a poor rebuild done previously and something with the assembly of the bottom end was done incorrectly. I shall split the cases tomorrow night and post what i find. I think i maybe needing some guidance, have just bought some plastigauge so i can check clearances.

[lukemillar]

If it was me and given how stripped the engine already is, I'd split the cases and investigate. It isn't a lot more work and at least you will either get piece of mind or find out the root cause of the issue.