Tappet clearance variation

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Joined: Fri Nov 28, 2014 11:02 am
Location: Australia

Tappet clearance variation

Postby stuart132 » Mon Mar 13, 2017 6:48 am

Hi, could anyone throw light on the following points:-
I adjust the R.H. Exhaust tappet to .006" with the piston at T.D.C. on compression stroke.
I noticed that when the piston is approaching T.D.C. (piston approx 1" from TDC) the R.H. Exh. tappet clearance is close to .012" and gradually decreases to .006" at TDC. This variation is only on the one tappet. When checking the R.H.Inlet and L.H. Cyl. Exh. & Inlet the varriation is only .001" (same procedure used).
If wear is the cause for the variation on the R.H. exh.cam, I find it difficult to understand why it should occur on the low side of the cam profile, may be I am missing something! or could it be caused by the cam follower? Do not want to strip the engine again have done so 6 months ago to replace the centre bearing so what other problems could this cause apart from increase tappet noise ?

I should mention that at some time in the past, (prior to my ownership) the engine was fitted with 'high lift cams'
so any idea where I could find the procedure to time high lift cams? I would like to check the timing just in case the mechanic timed it for std cams

Thank you any advice would be appreciated

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Joined: Wed Nov 18, 2015 7:43 am
Location: wiltshire

Re: Tappet clearance variation

Postby g5wqian » Mon Mar 20, 2017 7:58 am

high lift cams will be same as any other cams to set the tappets .
all you have to do is get cylinder on the firing position [when rockers are not rocking ] and that will mean the cam is on the clearance ramp and then you can adjust the rocker clearance on both exhaust and inlet on that cylinder .

then just go to next cylinder and do same and adjust that one .

be carefull of using any TDC device screwed down into the spark plug holes as if you try turning engine over with it dropped down in there you will end up with a valve wedged against it and problems , i find it best to use a strip of bright coloured plastic with marks made on it for tdc and firing position , that way you wont catch the valve of damage it .

it is a little difficult to probe for tdc on the twins due to angle of plug hole in relation to piston but a small torch proves usefull here to look down into hole to watch piston movement .

the .006" tappet clearance is a little fiddly to get exact but i found the easiest way is to either go for .008" first and then creep up to .006" or to lay an .006" feeler in the gap and adjust the rocker down to it , but a lot of time when you tighten the locking bolt it might reduce the clearance and require going over it a few times to get it just right .

in order to set the cam timing i guess youll need the specs for the cam as to where inlet and exhaust open and shut and point of max lift is and to also set a degree wheel up on the crank and set a datum pointer at TDC .

Posts: 58
Joined: Fri Nov 28, 2014 11:02 am
Location: Australia

Re: Tappet clearance variation

Postby stuart132 » Fri Mar 24, 2017 3:28 am

Thanks for the hints and information, I have set the tappets but still get the .010 clearance at 1" before TDC which reduces to .006" at TDC (only on the timing side Exhaust. valve) I will have to investigate the Exhaust cam on the next strip down!! as it looks like it has a worn spot.

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Re: Tappet clearance variation

Postby 1608 » Fri Mar 24, 2017 7:41 pm

My bet is that the follower has a hollow at the point where the can first makes contact with it. The cam will wear somewhat round the whole circumference, but the follower will wear a slight hollow just where the cam lobe first contacts the follower.

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