Oil pump pluger guide screw

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
Andy E
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Oil pump pluger guide screw

Post by Andy E »

Ref my 1956 AJS 16MS

There is an oil leak from the oil pump plunger guide screw. I have not yet removed the screw, but I cannot see a sealing washer under the screw head, and the parts book doesn't seem to show one. How is this screw supposed to be sealed? I don't want to just fit a washer if this would affect the depth of insertion of the guide pin into the pump plunger.

The manual states that you should never fit the guide screw without both pump end covers being removed. Does this just refer to a rebuild where you have removed the plunger? I had presumed that provided that I don't turn the engine over, the plunger would remain correctly aligned if I removed the guide screw.

Thanks for any advice.
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Pharisee
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Re: Oil pump pluger guide screw

Post by Pharisee »

There should be a red fibre washer under the screw head.
If you remove the guide screw, there is nothing to prevent the plunger from moving in the bore... not rotating, if you don't turn the crankshaft... but from sliding along the bore. You may need to remove the end caps to push it back to the position where the guide pin can be correctly located in the groove. If you take the guide pin out of the screw, make sure you put it back the right way round. The end of the pin with the reduced diameter goes into the screw, NOT into the pump plunger groove.

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Andy E
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Re: Oil pump pluger guide screw

Post by Andy E »

Hi Pharisee

Thanks for that, there Is definitely no washer fitted to the guide pin screw on my bike. I'm a bit surprised that's it's taken so long for the leak to develope. I'll remove the pump end covers and do the job properly.
Mick D
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Re: Oil pump pluger guide screw

Post by Mick D »

Hi

I'm not so sure a fibre washer was fitted here originally - my G3 didn't have one. Not to say it won't fix your issue but so might a good clean and a smear of Welseal.

Regards Mick
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Pharisee
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Re: Oil pump pluger guide screw

Post by Pharisee »

The crankcase is counter-bored to fit a fibre washer perfectly. Mine was fitted with one when I bought the bike so I replaced it when I re-built the engine.

If you take the end caps off, replace the thin paper gaskets with a thin smear of a sealant like 'Hylomar'. You definitely don't want an air leak, particularly on the return pump end. That will effectively stop the return pump from sucking oil up from the crankcase... as I found out to my cost.
The screws are 3 BA and are easily stripped, so use caution :)
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Mick D
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Re: Oil pump pluger guide screw

Post by Mick D »

Hi

The case would need to be spot faced anyhow in order to allow the screw to seat - I don't see any reference to a sealing washer in the parts lists, but as I say, fitting one may fix the issue, I just don't believe one was installed at build.

Regards Mick
zwarts
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Re: Oil pump pluger guide screw

Post by zwarts »

These engines never did have a fibre (or any other) sealing washer beneath the guide pin screw head. To seal the thread, a sealant such as Wellseal/Hylomar/Loctite etc. can be used. If the surfaces are not damaged, the spot-facing on the crankcase and the shoulder of the nut will provide sufficient seal.
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Re: Oil pump pluger guide screw

Post by Andy E »

I have some Welseal and pump end cover gaskets so I'll strip the pump out this week. I'm mystified as to why this leak would develope now though. I've ridden the bike for about 1000 miles and have not touched the guide screw which is tight in the crankcase.
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Re: Oil pump pluger guide screw

Post by shifter »

Forgive my ignorance so can the oil pump bolt/screw in the photograph above with the red fiber washer under it not be taken out just to replace/fit a washer without covers and plates being taken off
?
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Pharisee
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Re: Oil pump pluger guide screw

Post by Pharisee »

shifter wrote:Forgive my ignorance so can the oil pump bolt/screw in the photograph above with the red fiber washer under it not be taken out just to replace/fit a washer without covers and plates being taken off

When you remove that screw, the pump plunger guide pin will almost certainly come out with it. With the guide pin removed, there is nothing to locate the pump plunger. You may be lucky and it will stay where it is but you may also be unlucky and the plunger will slide along the bore. If that happens and you attempt to screw the pin back in again, you can damage the pump and/or the crankcase.
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