G3 oil pump guide pin screw/body

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Re: G3 oil pump guide pin screw/body

Postby tippi11 » Wed Dec 05, 2018 8:59 am

Good morning,
For somebody like me who grew up with metric sizes the provided British and US thread form details are good to know while also confusing due to the numerous variants.
What I did so far: I carefully honed down the thread peaks of the guide pin screw at the very top end on a length of about 4mm (uppermost thread section expected damaged). With these thread peaks off, it was easy to fully screw it into the crankcase hole. The screw now abuts against the crankcase as it should ...
Nevertheless I will buy taps in different sizes, e.g. 3/8" BSF 20 tpi, from Tracy Tools to finally "rebuild" the thread.
Thank you very much again.
Regards, Peter

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Re: G3 oil pump guide pin screw/body

Postby Yak » Wed Dec 05, 2018 11:20 am

The guide pin screw may have had a different use at some stage. I'm far from being an expert but wouldn't have thought alloy would blunt the steel thread. Just a thought.
Ray.

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Re: G3 oil pump guide pin screw/body

Postby tippi11 » Wed Dec 05, 2018 10:05 pm

Yes Yak,
That is a thought I had as well at the very beginning of the story - I have no idea "what" happened with the screw and/or the crankcase. I decided not to ask the previous owner (unsure whether he knows engine details or will tell me truth...).
Regards, Peter

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Re: G3 oil pump guide pin screw/body

Postby Dixter » Thu Dec 06, 2018 5:49 am

If you run a 3/8-20 tap in that hole, you will be able to lubricate the guide screw with tears. Find a decent ruler graduated in the Imperial system, ie Inches. Count the number of threads (the crests) on your existing guide screw equal to 1/2 inch. Multiply that number by 2. Order a tap with the resultant as TPI, threads per inch. I’m confident the tap will be 26 TPI.

Ciao, DC
Dick Casey
The RideNut

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Re: G3 oil pump guide pin screw/body

Postby clanger9 » Thu Dec 06, 2018 8:47 am

If your cleaned-up pin holder now screws fully home, I’m not sure I’d risk/bother with tapping the crankcase hole while the engine is in-situ.
Just screw it in, it’ll be fine! You can deal with the crankcase thread at the next rebuild...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: G3 oil pump guide pin screw/body

Postby clive » Thu Dec 06, 2018 1:08 pm

clanger9 wrote:If your cleaned-up pin holder now screws fully home, I’m not sure I’d risk/bother with tapping the crankcase hole while the engine is in-situ.
Just screw it in, it’ll be fine! You can deal with the crankcase thread at the next rebuild...


I agree, but then that's my attitude with all my bikes. When my fellow section member (and proper engineer) works on something for me he is appalled by the bodges, sorry I mean long term temporary repairs, he uncovers............ Just about to try fixing the exhaust leak on the car with exhaust tape, they still sell it but I had to get it on-line as all the DIY car repair shops round here have closed.
clive
if it ain't broke don't fix

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Re: G3 oil pump guide pin screw/body

Postby clanger9 » Thu Dec 06, 2018 3:52 pm

Yep. I’m all for doing the job properly... but if a proper job gets in the way of being able to ride the thing (and there’s no immediate danger of it breaking), then keeping the bike in one piece wins out for me every time!

Each to their own. If mechanical perfection is your pleasure, then that’s what you’ve got to do! :D
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: G3 oil pump guide pin screw/body

Postby Mick D » Thu Dec 06, 2018 4:29 pm

Hi

Are you sure the thread in the crank case has not been crossed? Accidently extending the crossed thread by tapping will compound you problems.

If you can't inspect the thread try leaving well alone.

Not wishing to teach you to 'suck eggs' but when you use a mirror and a flashlight it's best to bounce the light beam off of the mirror rather than try to shine it directly up the bore.

Regards Mick

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Re: G3 oil pump guide pin screw/body

Postby tippi11 » Thu Dec 06, 2018 10:44 pm

To be honest, I like clanger9´s suggestion ("Just screw it in, it’ll be fine! You can deal with the crankcase thread at the next rebuild..."), particularly at this stage of the project. More surprises other than the pin holder thread thing may come up because I dont know the quality and history of mechanical works from the previous owner. Worst case scenario could be an engine rebuild, will see what happens in the next weeks.
Regards, Peter

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