Topic: Rear shock tower/mounts threaded hold-down portions;threads getting worn..

[newday777]

Years ago I bought a CB750 that a previous owner had actually cut part of the threaded portion of the top shock mounts off, and there wasn’t enough”meat” left to install a decent pair of OEM shocks and install the standard dome nuts.

I cut the remaining threaded portion off, drilled and tapped them for 12 x1.25mm (standard thread for CB750 top shock mounts) and was horrified at how hard the original mounting posts were, I had to buy a special tungsten drill bit to drill the holes! I then just threaded them and installed some 12 x 1.25mm bolts into them and called them it good.

Terry,to clarify,you said you "cut the remaining threaded portion off,drilled and tapped them for 12 x 1.25..." ?
I don't understand:if you cut the remaining threaded portion off,what part did you drill and tap ?
I would imagine the remaining shock mounts O.D. were much larger than 12mm ??

Did you install 12mm bolts all the way through the shock mounts after drilling all the way through them ?

This will show you what Terry was talking about cutting off the threads to the hard shoulder on the mounts.

Terry
Yes indeed they are super hard. I dulled several drill bits in some depths, it changed hardness.

[Terry in Australia]

Sorry mate, on a CB750 the top shock mount is around 16mm OD, so drilling and tapping them for 12mm bolts left plenty of metal. No I didn’t drill them all the way through as I only needed to install a wide enough washer on each side to stop the shock from coming off. I probably drilled/tapped around 25mm deep. That allowed a 12 x 25mm bolt and a thick 22mm OD washer to hold each shock on. That bike is still getting around many years later, with no issues.

[grcamna2]

Sorry mate, on a CB750 the top shock mount is around 16mm OD, so drilling and tapping them for 12mm bolts left plenty of metal. No I didn’t drill them all the way through as I only needed to install a wide enough washer on each side to stop the shock from coming off. I probably drilled/tapped around 25mm deep. That allowed a 12 x 25mm bolt and a thick 22mm OD washer to hold each shock on. That bike is still getting around many years later, with no issues.

Ah,ok Terry, did you try drilling with a Cobalt drill bit and have any success ?

[Terry in Australia]

Sorry mate, on a CB750 the top shock mount is around 16mm OD, so drilling and tapping them for 12mm bolts left plenty of metal. No I didn’t drill them all the way through as I only needed to install a wide enough washer on each side to stop the shock from coming off. I probably drilled/tapped around 25mm deep. That allowed a 12 x 25mm bolt and a thick 22mm OD washer to hold each shock on. That bike is still getting around many years later, with no issues.

Ah,ok Terry, did you try drilling with a Cobalt drill bit and have any success ?

Sorry, you're right Bill, it was a cobalt drill bit that I had to buy. That original Honda steel was as hard as a rock.