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valve timing Model 18

Submitted by john_hawden on Wed, 29/11/2017
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2 questions chaps. I`m trying to set the valve timing on a 1946 model 18, there are no timing marks so I`m starting from scratch.

Q1, all books state set the valve timing as the valve just opens (inlet), closes (exhaust) I`ve set my dial gauge on the top of the pushrod to do this (the head is not on yet)...is this method OK.

Q2 Which is the most crucial, the inlet timing or the exhaust timing, I can get one or the other spot on between 25 and 30 degrees but not both.

cheers John

Submitted by andy_marks on Thu, 30/11/2017

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Hi John

In theory yes - the valves should start moving at the same time as the pushrods. Personally I'd rather get onto the valve directly if you can though.

As for Q2 it depends slightly on who you talk to. Some people say the inlet is the most important - so get that right and then worry about the exhaust. However others say it's more important to get the overlap evenly either side of TDC and the error about the same on both. Assuming it has the same number of teeth on the various gears as the post 48 models, and you have 3 key slots in the gear on the end of the crankshaft you should be able to get your inlet cam to within 6deg - but then the exhaust cam can only be set to 18deg intervals. That could be a long way out!

If for example you currently get your inlet to open at 27 but find you then have the choice of 20 or 38 on the exhaust closing, I'd use the 3 key slots to move both cams 6 degrees to get the inlet at 33 and the exhaust at 32. You may be better of having both out by 2/3 degrees than one spot on and the other 5 or even 8 degrees out.

Does that make sense? What figures are you currently getting?

Andy

Submitted by robert_tuck on Thu, 30/11/2017

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On the later twins it was always important for the inlet to lead the exhaust by 8 degrees,don't know how if any that relates to singles.

Submitted by john_hawden on Fri, 01/12/2017

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Thanks Andy,after a couple of days messing about (and much t`internet research) the closest i can get is 30 BTDC (inlet opening) and 20 ATDC (exhaust closing). Maybe a bit more messing with the 1/2 time pinion might balance the settings up a little, as you suggest. I did in fact mark the cams when i took them out, checking these the inlet opened at TDC ??, exhaust closed at 36 ATDC.

It must have run for ages like that, the bore was on 60 thou over.

I think these engines must have been(are) tolerant of a great deal of abuse.

Submitted by andy_marks on Fri, 01/12/2017

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Hi John

Does your half time pinion have the three key slots? If it does you could use it to retard both by 6 degrees you'll get inlet opening 24BTDC and exhaust closing 26ATDC. That's as about as good as they get!

Worth a try?

Andy

Submitted by robert_tuck on Fri, 01/12/2017

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My Ulster has timing 10 degrees either side of tdc with extra valve clearance so I would think Andys idea is good.

Submitted by john_hawden on Fri, 01/12/2017

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Previously Andy Marks wrote:

Hi John

Does your half time pinion have the three key slots? If it does you could use it to retard both by 6 degrees you'll get inlet opening 24BTDC and exhaust closing 26ATDC. That's as about as good as they get!

Worth a try?

Andy

cheers Andy, I thought of that too and thats the way I set it this afternoon. I`m glad you put that though. It gives me greater confidence that the machine will soon live again.

Submitted by george_phillips on Sat, 02/12/2017

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My little hard back "Book of the Norton" which covers your machine, John, notes that the Model 18 has the inlet opening at 5/16" BTDC which equates on a 100mm stroke to 33deg and the exhaust closes at 5/16" ATDC, so again 33deg. It doesn't actually state when the exhaust valve opens. Hope this helps? Can't help with how you achieve it though.George

 


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