Stripped Thread - Best Solution?

[John P]

One of the tapped holes in the top of the crankcase "bell housing" where the starter motor housing bolts has stripped.

Approximately 6mm of thread has stripped as a result of using too short bolts but this still leaves around 9mm of good full thread plus a further 5mm of thread on one side only where the tapped hole runs into the curved section where the flywheel runs.

I am wondering whether it's best to drill out the thread and fit a helicoil but, as standard, they are 1.5 x Diameter long so this would give me around 12mm of good thread.   Alternatively I did wonder about just using longer bolts which would be engaged with the 9mm of remaining good thread plus gain some additional strength from the partial thread below that.

I think I'm talking myself into the helicoil option but wondered what others thought.

I'm not very keen to add additional fixing holes either side of the existing as I have seen suggested elsewhere.

John.

[Terrytuned]

John a helicoil is a stronger thread than the original if you need a longer thread use a 2D helicoil Terry.

[Nick Turley]

Helicoils are great don't hesitate.

[Chris KC]

I must say if there's enough good thread remaining I wouldn't generally be in a rush to fix it.

In this application though I guess it's seeing some load, and I heartily concur with Terry & Nick, a (properly installed) helicoil will be stronger than the original thread.

An alternative would be to tap it oversize, screw (& Loctite) in an alloy or perhaps brass plug, and start again. OK if you have lots of meat around the hole, maybe not in this case. A helicoil would be easier and stronger.

[Steve Jones]

Helicoil has worked for me in this location. As Terry says, stronger than the original aluminium and this is something that takes a load from the starter motor when it's operated.

Steve

[David Stepney]

Yes. helicoil every time!

[Zetomagneto]

5/16 UNC Timesert. I suggest UNC as unfortunately you cannot get imperial Timeserts, but they are brilliant thread repair inserts imho better than helicoils.
Or make your own insert from a 7/16 BSF bolt

[Jamie]

"An alternative would be to tap it oversize, screw (& Loctite) in an alloy or perhaps brass plug, and start again. OK if you have lots of meat around the hole, maybe not in this case. A helicoil would be easier and stronger. "

This is what I did some years back on some my BSA C15 crankcase threads and they are holiding up perfectly. It is a good alternative to helicoils if there is not much load.

Jamie.

[AustinWood]

I would try the longer bolt fix to start with. Very simple and no harm done if it doesn't work.
BSF tread repairs are readily available.

[Colin Wilks]

Re Gene's recommendation of TimeSerts, they do sell "British" 1/4 BSF (1/4 - 26) in the US, although not other sizes. I've never used them but I could see these making a smart job for manifold studs.
http://www.timesert.com/html/inchsert.html