Tappets on a 1955 18s

AJSBSA · Post by **AJSBSA** » Mon Oct 03, 2005 11:25 am

I replaced the rocker cover gasket on my AJS 18s and set the tappets to the book as I normally do but they were quite noisy so I checked them again but no difference. So as an experiment I set them like I do my Sunbeam S8 this is when one valve is open I set other and the difference took me by surprise there is now no mechanical noise at all.
I seem to have full compression and the bike still ticks over like a Swiss watch, but I am worried that this could cause long term damage has anybody else tried this.
Cheers Steve

clive · Post by **clive** » Mon Oct 03, 2005 1:31 pm

I think the books advise that there should be no free play at top dead centre but that it should still be possible to rotate the push rods ie no loading. For the alloy heads you should be setting the gap with the engine warm. On the iron head it can be done cold I think. Hope that helps. Personally I have never managed to get rid of the noise. If there is any loading I would expect increased wear. Depends how much you use your bike as to whether this will prove a problem. 1000 miles a year and it will probably see you out without noticeable wear unless of course you are a junior as your forum title suggests!

Steve Martin · Post by **Steve Martin** » Mon Oct 03, 2005 3:36 pm

Hello Steve, a few thoughts, rather than firm conclusions: the tappet setting process for the S8 involves setting eg. one Inlet tappet when the other Inlet valve is fully open, since , obviously, the cams are set at 180 degrees to one another. That relationship does not hold for the Inlet and Exhaust cams on a single. On the other hand, if one valve is fully open it seem reasonable to assume the other must be closed, and the tappet at its lowest point. Assuming you've adjusted the clearance correctly, there shouldn't be a loading wear problem; but I can't see why doing it that way would eliminate the noise apparent when going by the book. Intuitively, I would go along with Clive's implied suggestion of a degree of loading, thus taking up the clearance responsible for the noise, but why the two setting methods give different results I can't say. If you were to set up the tappets anywhere but on the 'non-lift' portion of the cam, that would increase the clearance, and thus the noise, not reduce it. I haven't a clear enough picture of the relationship between the cams at their respective max' lift positions to see what effect that would have on the other tappet. It would be worth working out though. As to any ill effects of loading, it depends partly on how much loading there is. I'm guessing here, but besides extra wear on the tappet heads and cams, might there not be an effect from the valves not totally closing? But as you say, compression seems OK, and it works. An interesting issue, that I hope will attract some advice from those that know about these things! Cheers Steve.

TommoT · Post by **TommoT** » Mon Oct 03, 2005 10:45 pm

I think the reason for the manual to state TDC on compression stroke is that this is the only point where the pushrods are completely free of any pre-load. Elsewhere in the combution cycle the tappets are either on their way up or down the cams' quietening ramps, i.e. the gradual reduction from max to zero lift.

AJSBSA · Post by **AJSBSA** » Tue Oct 04, 2005 11:28 am

Hi
Thanks for all the replys and I believe I understand whats happening now. I think there must be a thou or two of play some where thats taken up when you set the closed valve with other fully open. I use the bike daily for work so I should encounter any problems fairly quickly. It was always said that the heavy weight single was a very quiet engine and now after 15 years of ownership I understand why.
Thanks for all the help.

Tellybelly · Post by **Tellybelly** » Tue Oct 11, 2005 10:34 pm

My 18S engine is quiet, very quiet, very, very quiet.............

Biscuit · Post by **Biscuit** » Tue Oct 11, 2005 11:53 pm

But what happens when you start it Graham?

Tellybelly · Post by **Tellybelly** » Wed Oct 12, 2005 8:57 pm

I wish I knew Alan !!

Current status: Replacement mag with fat blue spark aquired but still need advance AR unit (just missed one on ebay at £35.00, complete with most of SR1 !!).Barrel off for re-bore and hopefully weld build up of broken fins. Head will get skim and leadless valve seats. Also needs weld repair of hairline crack between edge of combustion dome and one head bolt hole to be weld repaired (not much meat there is there?). Carb being rebuilt.

One day Arfur will rise again !

Can I ask one question. Do you know how much can be safely skimmed from the head before the valves run out of space?

Cheers
Graham

Chazzyb · Post by **Chazzyb** » Sun Oct 16, 2005 10:19 pm

quote: Head will get skim and leadless valve seats.

Don't waste your money on lead free valve seats, unless they're shot anyway. AMC alloy heads don't need them is the general consensus. I've done 6K miles this year on my 18S with no hint of valve seat recession.