Impending doom?

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Apr 15, 2004
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I'm worried that my engine is about to blow. It's been making a fairly loud knocking sound on startup. It's a sort of metallic clattering type of knock, rather than the dull sound you usually expect from a dying bearing. As far as I can tell, it's coming from the left side of the crankcase. Maybe the main bearing, maybe the rod? The knock goes away when the motor warms up (five minutes or so).

What worries me is the bike may have been run for an extended period of time with low oil pressure, thanks to the oil pump seal that DPO forgot to reinstall at some point. It seems to have plenty of oil pressure now judging by the amount of oil leaking out of the head but the damage may have been done already.

Am I worrying about nothing or might the motor be about to go BANG in a big way? I hate to do a total engine teardown at the start of the season but that's better than having it self-destruct. :(

Debby, worried
Noises are sometimes difficult to locate and one always expects the worst. Is it possible something is loose in the primary? I bought a Triumph recently that sounded like awful engine noise in the bottom end. When I pulled the primary cover, the rotor nut was laying in the bottom and the clutch basket nut was very loose. In both cases the PO neglected to bend the tabs over on the washer. FWIW.

Does the noise or knock vary with engine rpm?
Could be piston slap sort of clang clang clang sound, quite noticable on cold idle can be quite loud also tends to get quiter as engine warms and pistons expand. caused by exccess piston clearance. Just cured mine of this with new pistons and cylinders, nice and silent now.
Good luck
I'll have to run it again and listen carefully. It very well could be piston slap because that does describe the sound pretty well. I guess that would be a lot better than a main bearing going bad. Will post back with results!

Debby STOP and have a serious look or your right BIG TROUBLE jinggle janggle possible push rods, knock and clang bearings. rods,pins bushings etc etc have a look. Roger
Hi Deb,

Sorry to learn of your engine noise, they sure can be puzzling and as pigpen pointed out, you tend to think the worst.

Personally, I like snakehips’ piston slap prognosis. I’ve had pistons slap in Triumph, Harley and a Plymouth Valiant engines. And despite the annoying sound, I’ve never experienced a catastrophic failure from this phenomena.

A piston slap will be most noticeable at idle and disappear at higher RPM. Also, a piston slap has shallow “clang” sound. In contrast, faulty rod and main bearings have a deeper sound that is most noticeable after the engine has warmed up.

If you suspect piston slap, you can easily pull the cylinder for inspection. If you find it’s bores are oversized then you’ve probably found the source of your noise. A re-bore and oversized pistons and rings should silence that diabolical piston slap.

Ok, I ran it again this morning and listened carefully. It's sort of a clanging/clattering type of noise, loudest at idle. It disappeared in three or four minutes when the engine warmed up (I timed it). Ambient temp was about 70 (hot day today, reminded me of California). After shutting it off I measured compression (dry). 140 psi on each cylinder. So at least that's good.

So maybe it's ok to run it for a while? And a top end job should fix things up although it sounds like I'll need new pistons and probably a rebore.

clanging noise down deep in the engine

Hi Debbie,

Could be a valve tappet hard face broken off. It's happenned to two of the local member's 850s. Check the rocker clearance and if it's gotten loose for no other reason, it's the tappet.

If indeed you have a piston slap, you may run the bike that way for several thousand miles without repercussions. However, you shouldn’t flog it, least you crack the piston skirt. Again, this assumes you have a piston slap; so if you have some other problem, obviously all bets are off. But it sure sounds like piston slap to me, owing to a sloppy cylinder bore.

Guess I should check the valve clearances to rule that out. Would be nice if that's all it was - easy fix (except for the broken off metal fragments floating through the engine?? No oil filter or sump screen on mine :shock: )

I got the old boy all washed and waxed and out for a ride today. Sounded and ran good once warm. So I guess I'll press on and try to go this season as is. Maybe no ton-up rides though (not that I was planning any!)

As always, thanks for all the advice!

What year is the bike? Didn't the '75 have a hydraulic primary tensioner? I very vaguely remember starting up a '75 after a rebuild and practically fainting from the noise until the primary tensioned up and everything was fine.

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