Curious gouges inside case

I haven't yet split the cases on my 72 Combat motor, but I observed an interesting thing tonight as I peered into the bottom end - there are deep gouges, about an inch in length, on the inside of the case (front and back) where the bottom of the left con rod would pass with each revolution. I took pictures, and hopefully it is clear enough to make out the detail. I only ran this engine briefly years ago when I first got this bike and put it into storage - so no real knowledge of history.

The crank and rods seem to turn free and smooth, and there is not any noticeable contact anywhere - as for noticeable wobble, there is about 15 thousandths of play between the rods and the shaft per the feeler gauge. I felt the underside of the rod assembly and I don't feel anything that would have created the gouging - but maybe that's just because whatever it was has been worn away? I guess I'll know more once the cases are separated...

Anyway, I found it curious and thought I'd toss it out there for comment and general fuel for making fun of my motor. LOL


-kjo

Front
https://drive.google.com/file/d/0Bwpts7 ... sp=sharing
Rear:
https://drive.google.com/file/d/0Bwpts7 ... sp=sharing

sometimes aftermarket rods can do that , and if they do its usually on the left side, not sayin thats what happened with yours.
Don

Don,

Interesting, good to know. I think these are original, pistons definitely are - although I just measured the piston skirts and they are both a few thou's under spec (2.870), so will factor that into my planning

Thx,

-kjo

hears my guess, in its previous life, crank/bearing failure, causing stretch and case contact with the rod bolts/nuts, maybe over revving,caught in time without blowing up , engine rebuilt with new stock stuff ( nothing else available back then) and is good now.

You may have something there - I was trying to visually determine if the bearings had been replaced, but they have no visible markings (so if replaced ended up with numbered side facing in?)- would be great if these had already been done (it is a Combat motor, so hopefully person doing the work would have known of bearing issue) and I didn't have to replace w/ a new set of C3's...

I reckon Jimbo's probably got it

I think if the rod or cap had made the mark it would be a continuous "scrape" making a much smoother arc in the case. A rod or cap may have originally been scraping the crankcase, but to me it looks like the previous owner has got out his grinder and attempted to increase the clearance so that nothing will scrape in future. If you look at the second photo there are 5 or 6 gouges and they are rough as if something has ground into the crankcase. Perhaps it was done to clean up the area where something came loose and scraped the crankcase but was caught before it became catastrophic.

Turning the crank carefully while supporting the rods should show if anything is scraping the crankcase. Is the mark on both drive and timing sides?
David

I think you will find the inside of the cases are not perfectly round so when a con rod nut does come loose it will scrape only where it can touch. Happened to me so I know.
Dereck