Model 18 Timing/Tappet adjustment

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Rupert123
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Model 18 Timing/Tappet adjustment

Post by Rupert123 »

I wonder if you could help with some information regarding ignition timing and tappet adjustment ,when doing the timing, rod down plug hole up to TDC compression stroke rotate both pushrods to verify correct TDC ,unfortunately inlet push rod does not turn exhaust does how can i tell that i am at correct TDC .
This points to tappets out of adjustment looking on the forum three small thin spanners are required as my tool collection is small at the moment regarding imperial sizes what size spanners are required for the undoing locknuts.

Kind regards,
Martin.
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Pharisee
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Re: Model 18 Timing/Tappet adjustment

Post by Pharisee »

If you can rotate the exhaust pushrod at your TDC, then you are indeed at TDC on the compression stroke. At TDC on the exhaust stroke, the exhaust valve would be open and you wouldn't be able to rotate that pushrod.
If you can't rotate the inlet pushrod at TDC on the compression stroke then the pushrod needs to be adjusted.
I've got some spare pushrods in the garage. I'll check the nut size later this morning if somebody doesn't tell you first.

EDIT...
Just checked an old pushrod... The lock-nut and the hex on the top of the pushrod required a 3/16" Whit / 1/4" BSF spanner. The screw cup needs a 1/4" Whit / 5/16" BSF spanner.
Last edited by Pharisee on Sat Apr 08, 2017 11:21 am, edited 1 time in total.
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ajscomboman
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Re: Model 18 Timing/Tappet adjustment

Post by ajscomboman »

You'll need a couple of 3/16 Whit or 7/16 AF spanners and a 1/4 Whit to carry out the adjustment.
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Rob Harknett
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Re: Model 18 Timing/Tappet adjustment

Post by Rob Harknett »

You will need one of the larger spanners to be thin, to hold the bottom nut, when unlocking the top nut of the two. Look out for a "Terrys" spanner to hold that bottom nut, they are thin and good quality steel. Adjusting the gap is usually easy. Oil keeping threads lubricated should make it possible to just use finger and thumb. You should also be able to use your fingers, prior to just nipping up the lock nuts. This procedure can often change the gap set. So always recheck the gap. If the manual stated the gap should be set with the engine warm. Why not start the engine, warm it up and check again.
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thornebt
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Re: Model 18 Timing/Tappet adjustment

Post by thornebt »

Pharisee wrote:If you can rotate the exhaust pushrod at your TDC, then you are indeed at TDC on the compression stroke. At TDC on the exhaust stroke, the exhaust valve would be open and you wouldn't be able to rotate that pushrod.
If you can't rotate the inlet pushrod at TDC on the compression stroke then the pushrod needs to be adjusted.
I've got some spare pushrods in the garage. I'll check the nut size later this morning if somebody doesn't tell you first.

EDIT...
Just checked an old pushrod... The lock-nut and the hex on the top of the pushrod required a 3/16" Whit / 1/4" BSF spanner. The screw cup needs a 1/4" Whit / 5/16" BSF spanner.
After rebuilding my G80 engine I turned the engine over until I could rotate both pushrods. I then turned it back until the piston was the required distance before TDC. I've started the engine since then but haven't ridden the bike as I still have a couple of bits to complete it. So, is my thinking correct that you should be able to rotate both pushrods at TDC? Cheers. Bruce.
Rupert123
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Re: Model 18 Timing/Tappet adjustment

Post by Rupert123 »

Thanks everybody for all the info,
It is much appreciated have ordered the spanners of ebay to get the tappet adjustment done including the Terry flat one, what i was quite surprised at were some of the prices people were charging for these old spanners i suppose i you need them you buy them ,many moons ago i left my job took the tools i used {all imperial} with me, the new job required metric tools only as well as that i started riding Japanese and italian bikes so i had two ammo boxes full of tools i did not use could not sell or give away when moved house all these tools were dumped up the local tip if i only new then what i know now.

ATB,
Martin.
Mick D
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Re: Model 18 Timing/Tappet adjustment

Post by Mick D »

Hi

When you set the engine to it's timing position it is advisable to always turn the engine in the direction of normal rotation. This ensures any errors due to backlash are eliminated.

So, find TDC, wind the engine back beyond the desired timing angle and then rotate it forward to the timing angle.

Regards Mick
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