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MG Midget and Sprite Technical - damperclamp bolt size

Can someone tell me the size for the front damper suspension arm clamp bolt. In particular, the length? The one that goes into the end of the suspension arm, to clamp and stop the upper trunion bolt from turning.

When I troed to fit a bolt I found that there is the stub end of a sheared off pinch bolt already there and I will need to drill it out and re-tap. But what depth sould I be drilling / tapping to? Is there a danger of going too deep and weakening the suspension arm?

is it item 2 on the following link if so then Brown & Gammons list it as being superceeded as shown in the second link (3/8 unf x 4" long)
Kevin Fuller

Sorry obviously reading all of the post is not my forte!! having read it through properly I assume that it is item 5 in which case I am unable to help
Kevin Fuller

It's item 9 from what I understand IIRC 5/16" UNF but it has been awhile. Guy you should be alright to drill it out and you should feel when it breaks through the far end of the bolt as the bolt won't fit right to the bottom of the tapped hole.
David Billington

Just had a look at my collection of 4 or 5 and I have the same problem! However one looks clear (no bolt in it) and the depth is around 1 1/2".
Bill Bretherton

If you use a left hand drill bit you may be lucky and it'll grab and unwind the screw saving the trouble of trying to pick the remnants out. A neighbour of mine called a local engineering firm about where to buy them and they thought he was taking the piss as they had never heard of them so they learned a technique that day, another local fastener supplier stocked a range of left hand drills and so he came back later in the day with a few for me to use and 1 out of 3 broken screws unscrewed out but these screws had been loctited.
David Billington

Kevin, no not 2. and not 5. Third time lucky? - its item 9! ;-)

Good point David, the bolt will stop short of the hole bottom so a pilot drill shoud break throgh and give the total depth.

Since what I should really be doing is clearing the remains of the bolt from the existing thread, rather than enlarging it, what tapping clearance should I be aiming for?

Incidentally , that Moss drawing appears to show it as a screw when I assume it should be a bolt with a shank part which should be designed to snug into the trunion bolt notch.

Just been out to the garage to investigate further. I hadn't spotted there was a stub of a bolt in there as it has sheared about 7mm down. At least I though it had but now I have cleared the muck out I see it has been drilled down to that depth.

Then I see there is a small, shallow pilot hole, about 2mm dia. But when I tried to drill it deeper the drill just isn't cutting. I'ts definately not what I want, but I think there is a broken off drill bit down in there. So that's going to be a real problem ! Grr!


My Presto book gives the tapping size for 5/16" UNF as 6.90mm. If a broken bit in there then the left hand twist drill would be my first thought but IIRC I've seen recently some hard drills advertised somewhere for drilling out hard items such as HSS bits. Can't remember where at the moment. I have on a few occasions made a custom hollow bit for cutting around such things but I have the capability to do that.
David Billington

The outer part of the original bolt has been drilled out completely, down as far as the slit in the clamping part of the suspension arm. So nothing to get a hold on there. Then the 2.5mm pilot hole but with a broken off drill bit maybe 1mm below the surface, so again, nothing to get a hold of or for a left handed drill to bite into.

I'm stumped!

Guy, I presume sheared off pinch bolts are common in the front shocks. They rust in and it's difficult to get penetrating fluid in there. As I said, I have the same problem, well for one side at least, and new units (not exchange) are expensive.

I'd have thought it's virtually impossible to drill out a drill bit.
Bill Bretherton

You can get diamond core bits off ebay quite cheaply in that size range so might be worth a go, you'll have to peck at it and flush the debris frequently but the hardness shouldn't be a problem. Is the arm removable? I've run across many where the arm was held on with a nut and probably just as many where it was welded on with with a ring of weld where the nut would bear, never tried to remove an arm held on with a nut though so I don't know how tight a fit they're on the spline but a swap may be possible if the main unit is good.
David Billington

The only thing I can think of trying is to drill a ring of holes outside, but adjacent to the offending drill bit. I have no idea how deep it goes but given its only 2 or 2.5mm diameter there should be enough space. Maybe worth a try at least.

If I can get it out by doing that I am not sure how I then get on with re-centering a hole sufficiently accurately.

Bill, I have encountered rusted in pinch bolts before but have always got them to move by application of a bit of heat. Its always worth a try early on and before going to far with the brute force approach that so easily results in a sheared of bolt head!

Maybe something like this just need to find in the UK.
David Billington

I remove broken drill bits using a MIG welder - a little spot of weld on the end of the broken drill and it shatters - repeat until it it is all out.

Welding down a narrow hole demands a steady hand!
Chris at Octarine Services

Guy, yes I guess it's always wise to apply heat but, of course, it's also easy to be impatient!
Bill Bretherton

Another challenge there, Chris! I might give that a go tomorrow.

I suspect, though I am not sure, that I might have a spare pair of dampers buried somewhere in the garage. How easy is it to R & R the top lever arms to the damper spindles?

"I presume sheared off pinch bolts are common in the front shocks."

Bill, you are correct. It is a common problem.
Dave O'Neill 2

The original bolt had a smooth section that contacted the pinch part of the shock arm. I purchased replacements from Moss USA and received bolts that were threaded all the way. I could have gotten them at the hardware store. I will be looking at AN fasteners that should have the correct smooth portion. I don't know how much the extra threads would wear the end but would rather not find out. Maybe another supplier over there would provide the correct bolt.
J Bubela

Yes, they clearly should be bolts, not set screws.

My problem was resolved by finding that l have a box of 6 spare damper units, none with sheared off clamp bolts. They just needed looking for!

This thread was discussed between 26/08/2017 and 29/08/2017

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