Topic: Mystery piece

[M Schepis]

Decided to take my oil pan off to inspect filter and check tensioner/chain condition. Besides some crud on the filter and what looked like pieces of clutch friction material I also found the attached aluminum remnant, it looks like a piece of a fin. Does anyone have any ideas what this might be?

Also, using a vernier I measured to the indicated point on the primary drive tensioner and got a reading of 69mm. I understand that at 70mm or above the chain should be replaced, what are your thoughts on this?? The more i look into this engine the more I'm getting discouraged. The odometer reads 22000 miles, if this can be believed,  it seems early for a primary chain to have so much stretch?

[bryanj]

22,000 depends on how it was ridden and its still old.
The piece may be from a casting inside the crankcases

[scottly]

That piece may have come from the oil pan itself. I seem to recall seeing pieces like that before?

[M 750K6]

Have you checked behind the sprocket cover. Chains can break and punch a hole. I've seen pictures of JB Weld repairs, but I don't know how you'd remove any pieces that go inside the case. Hopefully, that's not your issue.

[HondaMan]

If this is a CB750: there is a small blade-shaped casting in the bottom of the engine that helps hold the cam chain on the crankshaft when the top of the chain is loose from the cam sprocket. This was most likely done to facilitate building the engine at Honda, so they didn't have to spend a lot of time fishing around or re-opening the crankcases (and re-cleaning the sealant, etc.) if it fell off when someone was setting up the crankshaft bearing shells in production. Now, if the chain got real slack because someone didn't tune up the cam chain tensioner when it got loose, the chain can loop its looseness over the sprocket during a sharp deceleration, and that link of chain snaps off the top edge of the little blade in the bottom of the crankcase. In roadracing, this happened to almost all of the engines I saw.

In real-life riding, this won't make any difference (mine has been gone since 1972, more than 150k miles ago) so long as the cam chain tensioner is set once in a while.

If the cam chain is sloppy/loose, and the engine hasn't been run in a long time, it will make the engine sound like it is about to grenade, especially at idle. A full tuneup will usually make it much happier: a few hundred miles of new riding will make it even more so. These bikes HATE being parked.

[M Schepis]

Thank you to all, I feel a little better. Still waiting for my clutch to arrive so I can do just that, put some miles on this
machine!!
BTW any thoughts on the 69mm reading for the primary chain condition?

[HondaMan]

Since 70mm is the Honda wear-limit spec for the primary chain's measurement, I'd suggest using heavier oils for this season and maybe change out the chains for the next one?

In order to avoid an oil thread, I'll just recommend using 20w50 non-detergent oil, here...

[PeWe]

CB750:
I have seen that piece too
It sit beside the primary chain tensioner M6 bolt holding it in case.

Can happen during assembly when cam chain is down + rotate the crank before chain is around cam sprocket.

Here a C750 case where this has happened too. The tip is broken off like your piece found in an oil pan.

Yes for 20W-50 motorcycle oil stamped MA.
MA2 works too, especially when clutch need a better bite.

[bryanj]

Thats the bit i was thinking of just dont do many 750 now

[M Schepis]

You guys are an incredible wealth of information! Thank you all.

[Don R]

 You are not the first to ask about that tab breaking nor the first to ignore its demise. It's better removed than sitting in the pan, otherwise no big deal.