Timing Gear Shims

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Timing Gear Shims

Postby nomisimon » Mon Jan 11, 2021 5:33 pm

Hello...

I've been gradually fixing odd niggles on my 1953 G3LS, the last being a noise from the timing area. The inlet push rod adjuster cup was very slightly bent over and replaced but the noise remained; it only occurs when the engine is turned over with the inlet pushrod in place.

I took the timing cover off and found a rather knackered shim...see picture. This missing piece was in the bottom of the compartment. There is an intact shim on the exhaust timing gear and the spares lists only list shims for the exhaust valve gear.

The gears and cams are fine, as are the followers...there's a tiny bit of play on the exhaust follower but it isn't a problem.

I've noticed there is an oil slot on the exhaust gear bush but not on the inlet bush...possibly because the former has a sprocket on it.

My question is, is it OK to fit a shim on the inlet gear shaft...on the outer end where it goes into the cover?

Thanks

Simon

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Re: Timing Gear Shims

Postby Pharisee » Mon Jan 11, 2021 6:06 pm

Shims are available in three thicknesses, 0.003" , 0.005" & 0.010" , from AMC Classic spares. It's the exhaust camshaft that drives the magneto and with the cover in place, end float can be measured and shimmed if necessary. With the cover in place, the inlet camshaft is inaccessible and end float can't be measured. It looks like some previous owner has put a shim on the inlet camshaft when it didn't need one and put an end loading on the camshaft, eventually destroying the shim.
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Re: Timing Gear Shims

Postby nomisimon » Mon Jan 11, 2021 6:51 pm

Thanks John...that makes perfect sense.

I noticed the exhaust camshaft was moving in and out a little before I removed the cover...I'll get a selection of shims after I measure the endfloat.

Cheers

Simon
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Re: Timing Gear Shims

Postby nomisimon » Mon Jan 11, 2021 7:04 pm

I don't have the cams in the house with me but, which side of the gear do the shims go on?

Mine were on the outer end, and may only fit there. Is that correct or do they go on the inside, nearest the crankcase?

Thanks

Simon
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Re: Timing Gear Shims

Postby Andy51 » Mon Jan 11, 2021 7:43 pm

Hi, I fitted shims to my 18S, on the outside as I believe is right. As Pharisee says, you can measure the exhaust cam easily. To get the inlet cam right, I assembled the timing case with the inlet cam only, and fitted shims until they were too high and the case wouldn't fit home. Measure the gap between crankcase and timing case with feeler gauge, and remove shims accordingly- plus a couple of thou for luck, as a touch too much endfloat is better than none! Reassemble with exhaust cam and shim that as well, and Robert is your Uncle. Only takes a couple of hours. Andy

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Re: Timing Gear Shims

Postby nomisimon » Mon Jan 11, 2021 7:55 pm

Thanks Andy
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Re: Timing Gear Shims

Postby Greybeard » Mon Jan 11, 2021 8:33 pm

Endfloat on the inlet cam can be measured accurately but it involves a complete strip down and reassembly of the cams in the timing case, with the gasket once the crank is removed. You can reach the inlet cam through the main bearing and measure the endfloat with a dial gauge from the inside of the crankcase.
It seems that endfloat isn't too critical (so Andy's method is fine if a full stripdown isn't on the cards) but if it can be kept to a minimum the engine will be quieter. When I took my 350 apart, there was only one shim (in similar condition to that of the OP's though still in place) and it was on the outer end of the inlet cam. The cams had an endfloat of 30 and 35 thou. It's now down to a more reasonable 3-4 thou so I'm expecting some silent running ;)

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Re: Timing Gear Shims

Postby SPRIDDLER » Mon Jan 11, 2021 8:45 pm

nomisimon wrote:
I've noticed there is an oil slot on the exhaust gear bush but not on the inlet bush...possibly because the former has a sprocket on it.

The zorst bush has an internal scroll/helix designed to prevent oil creeping along the camgear axle from the crankcase into the timing chest and will also 'screw' back into the crankcase any of the oil that returns down the pushrod tubes into the timing chest if it has bypassed the timing chest drain hole.
The inlet bush is blanked off so no oil can exit from the crankcase into the timing chest.
Last edited by SPRIDDLER on Mon Jan 11, 2021 10:13 pm, edited 1 time in total.
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Re: Timing Gear Shims

Postby nomisimon » Mon Jan 11, 2021 9:57 pm

Thanks gentlemen.

I'm going to order a selection of shims...I should be able to sort it out that way. I'll double check nothing else is awry before ordering.

Cheers

Simon
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Re: Timing Gear Shims

Postby Dixter » Wed Jan 13, 2021 7:48 am

Simon, determining the required thickness of the shims is eased with a piece of wire solder as used in electrical work. Cut a short piece, trap it where the desired shim would go, replace the cover, tighten the screws and crush the solder. Measure the thickness.

The British Army published a workshop inspection standards manual for your in motorcycle in 1953, available here.

http://archives.jampot.dk/book/Workshop ... ndards.pdf

The desired end float is shown in this image from the above manual.

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DC

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