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Just stripped a 150 Special engine casing ready for a rebuild. Everything in good order except a previous owner replaced the endplate studs with ones that were too long and which caught on the rivets of the rear sprocket.

This means that I need to repair 2 of the threads for new studs.

As this is a stressed item, what is the strongest way to repair this please?

I’ll be getting it done professionally. I just wanted to be able to speak knowledgeably to the engineer

Timeserts, have a look at these, close to you Paul

http://www.threadrepairkits.co.uk

http://www.threadrepairservices.co.uk

Yes these M7 Timesert inserts are a good solution here but you need the costly install kit and other right gear to get them in straight and true. Worth having if you repair a lot of gearbox endplates.
The self tapping versions of these inserts would also be a good solution here but M7 self tapping thread inserts are like hens teeth.
The type of repair is dependent upon how much damage has been done to the aluminium around the knackered thread.

Beedspeed do an M8/M7 endplate repair stud which in most cases is the easiest way to go. These only go in 10mm though. The back holes of the endplate can take longer studs.
If memory serves.......yamaha R1 uses M8 to M7 exhaust studs which will be longer and can be cut to suit.

I prefer to convert the damaged M7 thread to M8.
I make my own studs from ground M8 high tensile bar but you can consider using M8 high tensile car exhaust studs. They need to be the "cut thread" type and not the rolled thread type. The unthreaded part of the stud should measure 8.0mm nominal.

Remove the damaged M7 stud(s).
Fit the end plate.
Using the hole in the endplate as a drill guide, drill down deep enough with a 6.7mm drill bit.
Remove endplate and tap hole(s) to M8 x1.25mm.
Drill out the endplate stud hole(s) from the standard 7.4mm to 8.0mm. (best done on a radial drill or a mill.) I use a 4 flute slot drill to make sure the hole is true to size and not oval.
Check that your new stud will just pass through the hole. The stud is now acting as a dowel as well as a fixing.
Secure with a new m8 half nut that is 12mm A/F and shim washer.
M8/12mm A/F full nuts are easier to source. Use these and file them down to get the necessary clearance from the clutch sprocket.

MB sell a stepped stud.

Thanks very much. I’ll go with the stepped stud option.

This is by no means a contradiction, but my preference is to repair the damaged holes to return them to the best interpretation of the original design intent.

Having witnessed the stepped stud repairs fail (no doubt because they were poorly executed) I avoid that type as the M8 x 1.25 portion can pull out of the crankcase but the engine builder or rider be unaware, as the endplate stops the stud pulling all of the way out, as would be the case with an OEM, M7 x 1.0 fastener.

Wire thread inserts (Helicoils) on the other hand achieve the same purpose as an oversized thread repair whilst allowing the original size fastener to be utilised. Bear in mind that they are fitted as a standard feature in many alloy engineering components from the point of manufacture, not just as a means of repair.

However, often even their use is not necessary, as the damaged hole can usually be deepened & tapped to take a much longer M7 x 1.0 stud than those intended, to take advantage of the increased strength via more thread engagement. Suitable longer, rolled thread studs are plentiful by looking elsewhere for their use on Lambretta or V*spa, & I would advocate their use even on a 'standard' engine build where the crankcase is of significant age.

Whilst I'm @ it, although even some dealers have little concept of the need, proper, hardened M7 nuts & springwashers should be used & not the cr@p items so often sold.

For what it’s worth I repaired one of my end plate threads using a bog standard helicoil. Good solid repair and it’s always held , been on and off a few times torqued to 17nm good to go.

On the other hand i used a Beedspeed (yes I know sorry) stepped stud m6/ whitworth something on my chain case, pulled out pretty much straight away, fixed it using a helicoil and it’s always held.

Here is a photo of one of my modified endplates which uses 4 countersunk M8 nuts to gain extra fixing strength for high powered motors.
Never had one of these come loose.

Storkfoot wrote:Thanks very much. I’ll go with the stepped stud option.

This is what I did. End plate fits well, I think. The true test will be when I build the engine properly.