Topic: Damaged engine case threads.

[VTCBike750]

Wondering what you guys would do?
1972 CB750

When I got my bike the PO had somehow damaged the threads in one of screw holes on the engine case that holds the drive sprocket cover. I wasnt able to run a thread repair tap to reform the threads, so I gently ran a cutting tap through the damaged threads. It holds well enough. I think drilling it out and adding a helicoil or timesert would be completely overkill.

Question. Is it worth adding loctite or loctite thread repair. Has anyone used loctite's thread repair?  This is a bike I'd like to restore

Thanks for the advice.

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[HondaMan]

The rear screw on the sprocket covers will misalign if the sprocket cover is from a different engine. For example: K0-K2 engines were all cast the same, in the same molds, as were their side covers (like the sprocket cover). Partway thru the K3 era, there were new injection molds made, first for the side covers and then, by the K4, for the engine cases. These new covers were about 1mm different in hole placement of the sprocket covers, in particular the rear one. If the upper screw is inserted and tightened in one of these covers before the rear one is, there will often be about 0.5 to 0.75mm misalignment, tilting the rear screw. If the owner then just muscles that screw in, it either strips or damages the case threads, or can even break the whole boss right off the back of the engine(!). I get to make new ones on these engines a lot: 4 times in this last year, for example, because the top of the boss was broken off and missing entirely.

[VTCBike750]

When I got the bike the original sprocket cover was cracked. I bet the PO tried to bolt on any 750 cover to replace it, realized they were starting to damage the threads, and stopped. At the time I noticed that the cover had a little notch at the top, not straight across like other covers I had seen on ebay. Ive been able to find what I think is the correct cover.

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[VTCBike750]

The rear screw on the sprocket covers will misalign if the sprocket cover is from a different engine. For example: K0-K2 engines were all cast the same, in the same molds, as were their side covers (like the sprocket cover). Partway thru the K3 era, there were new injection molds made, first for the side covers and then, by the K4, for the engine cases. These new covers were about 1mm different in hole placement of the sprocket covers, in particular the rear one. If the upper screw is inserted and tightened in one of these covers before the rear one is, there will often be about 0.5 to 0.75mm misalignment, tilting the rear screw. If the owner then just muscles that screw in, it either strips or damages the case threads, or can even break the whole boss right off the back of the engine(!). I get to make new ones on these engines a lot: 4 times in this last year, for example, because the top of the boss was broken off and missing entirely.

Once again a wealth of knowledge. Thanks Mark.

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[minimo]

What do you have against Helicoil?
I’ve had to do it for a few of mine and friend’s bikes and it works great.

My engineering pals all say that where they work, all aluminum parts that have screw threads are required to be Helicoil’d to mitigate galling that may occur between the different metals used as fasteners.


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[VTCBike750]

What do you have against Helicoil?
I’ve had to do it for a few of mine and friend’s bikes and it works great.

My engineering pals all say that where they work, all aluminum parts that have screw threads are required to be Helicoil’d to mitigate galling that may occur between the different metals used as fasteners.


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I dont have anything against helicoils. Ive used them before. Its more of applicability in this case. It seems overkill in this scenario.

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[HondaMan]

When I got the bike the original sprocket cover was cracked. I bet the PO tried to bolt on any 750 cover to replace it, realized they were starting to damage the threads, and stopped. At the time I noticed that the cover had a little notch at the top, not straight across like other covers I had seen on ebay. Ive been able to find what I think is the correct cover.

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You're very perceptive! That's how to tell them apart.

[VTCBike750]

I knew something was up when an ebay one didnt look the same.