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MG MGB Technical - Starter motor bolts - neither fit, which to use?

I'm putting an engine in my GT and the bottom starter bolt is holding up the process. The bolt that came out of the engine that was in there (both 18V blocks) looks to have far to coarse a thread (matches that of the top bolt except shorter length) and wasn't going to go in without a fight.

I looked up part # and found it matches that of the engine backplate top bolts. I used one of those but it was also reticent; I got fed up and I persuaded it and the result can be seen in the attached image. The bolt now runs ok in the hole but I'm not sure the threads are going to hold when I tighten against the starter.

Can anyone advise if the damage on the threads suggests the coarse bolt is the way to go? Any better ideas before I wreck the backplate?
Many thanks

Steve Postins

GHF105 "correct" bolt

Steve Postins

Hi Steve,

All the 4-synchro backplates I've seen were tapped with the coarse (UNC) thread, so I guess the coarse bolt's reluctance was just due to grit? It's a blind hole (gearbox casing behind it) so if you want to try to run a tap into it, grind the end of the tap flat.

Hope winding the fine bolt in didn't damage it too much!

hth,
--
Olly
Oliver Stephenson

Cheers Olly, I guess it has to be the other one but in my defence it felt like a real no go, even with Plusgas on the threads. Here goes!
Steve Postins

I would definetly run a tap into the trans. Otherwise, you run the risk of stripping the housing or breaking the bolt off. Niether a pleasant prospect. RAY
RAY

I didn't have a tap to hand (I've been meaning to get a proper set for 10 years now) so repeated my impatient work on the coarse bolt. It went in and is tight, but it took more effort than the other bolt and the threads are just as flattened. Fortunately the bottom bolt only runs through the backplate to fresh air next to the flywheel so the bellhousing isn't an issue. I'll keep an eye on it and will sort it out properly the next time I pull the engine.
Many thanks,
Steve
Steve Postins

AF tap and die set finally ordered to atone for my poor work yesterday! Every cloud...
Steve Postins

Steve,

It should be the UNF (fine) bolt - not the UNC (coarse) one.

The coarse top one runs thru a clearance hole in the backplate and screws into the alloy bellhousing. The fine bottom one screws directly into the steel backplate.

My guess is that the backplate threads are damaged, retapping them 3/8 UNF will clean them up, but use a new bolt to get max thread engagement and don't overtighten it. Fortunately most of the weight of the starter is carried by the long top bolt.
Chris at Octarine Services

This thread was discussed between 04/06/2009 and 05/06/2009

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