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Comet Valve Timing

redbloke1956

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Non-VOC Member
I am about to replace the Cam spindle, large idler and small idler on my Comet so I thought I would try and get my head around the timing marks on the various gears.
I have turned over the engine until the two marks on the large idler line up with the two marks on the cam gear, at this point however the other marks do not line up and the engine is near BDC, if I keep turning the engine over can the two marks coincide at another time (some revolutions later)or is this a once in 79 rotations thing?

Thanks in advance
Kevin
 

Len Matthews

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The problem arises if at some time in the past the Half Time Pinion has been replaced from one from another bike. These were individually stamped when the engine was first built when new. This means the marks may not apply to your engine. To overcome this problem, position the piston at TDC, align the double dots on the Cam Pinion and the Large Idler. Offer up the Half Time Pinion in such a way that the keyway in the pinion aligns with the keyway in the Mainshaft and the teeth meshes with the Large Idler without moving anything. Hopefully this should be somewhere near correct. The old dodge "Valves rocking at TDC" is a way of checking. All this assumes that the Cam Pnion has been pressed on correctly bearing mind that it's only an interference fit and slipped Cam Pinions have been known.
 

nkt267

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You can turn the engine over forever before the marks re-align and new idlers do not neccessarily have any marks. A quick dodge is to forget the marks for now. Degrease the idlers, cam and half time pinion(mainshaft pinion), and mark with small paint marks all the points where the gears touch not forgetting the 2 marks needed for the halftime pinion( 1 for the key and one for the large idler engagement) and take a photo of the lot.
When you have fitted your new spindle refit your old parts(before you fit the top end) and check the alignment, if you are happy with the setup lock the cam and mainshaft, remove the large idler and fit the new one.
The other option is to re-do the timing as per Richardsons and mark the pinions with and engraving tool afterwards. John
 

Tom Gaynor

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I endorse this. Even if the half-time pinion has NOT been replaced (and it might have been to take up backlash), seeing the punch marks is NOT easy. And even if you do get the dots lined up, and I did, when you need to align them again it's a lot easier to see paint marks than dots. Somewhere in the literature there is a calculation of the number of turns before they line up again. About 140, I think. I lost count, but it's a lot.
An easy check that your valve timing is correct is that both valves should have equal lift 4 - 6 degrees BTDC. Neville Higgins called this "timing on the rock" - MPH 648 I think.
You can turn the engine over forever before the marks re-align and new idlers do not neccessarily have any marks. A quick dodge is to forget the marks for now. Degrease the idlers, cam and half time pinion(mainshaft pinion), and mark with small paint marks all the points where the gears touch not forgetting the 2 marks needed for the halftime pinion( 1 for the key and one for the large idler engagement) and take a photo of the lot.
When you have fitted your new spindle refit your old parts(before you fit the top end) and check the alignment, if you are happy with the setup lock the cam and mainshaft, remove the large idler and fit the new one.
The other option is to re-do the timing as per Richardsons and mark the pinions with and engraving tool afterwards. John
 

davidd

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VOC Member
Tom's point is a very useful one. When in doubt, equal lift can occur at TDC of the exhaust stroke or before. The diagram in Richardson where all the marks line up is four degrees BTDC of the exaust stroke, and although it is not stated in the text, the valves in the diagram would be at equal height.

David
 

Howard

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If you're happy with the way it runs, and you're only replacing the idlers, use John's paint dots technique (I use felt tipped pen, it dries quicker), the position of the idlers doesn't matter, as long as the half time pinion, cam pinion and ATD pinion are right, relative to each other. Oh yes, and breather pinion.

If you accidentally let the pinions move when you take out the old idlers, the paint marks will still be useable, just make sure the marked teeth are both in full mesh with the idler.

H

ps. make sure valves are closed when you make the marks, otherwise (assuming like me you don't remove the pushrods) when you pull the idler out, the valve springs will turn the cam shaft and you'll lose your settings.
 
Last edited:

redbloke1956

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Non-VOC Member
Much thanks to all for all the info, I have been reading everything I can lay my hands on...Richardson etc and was getting a little confused but I think I am on the right path now, I wanted to be sure of everything before I attempt to replace the Cam spindle.I am a little concerned though about the possibility of one (or more)of the gears running slightly eccentric because as I rotated the crankshaft I noticed that in some positions the cam pinion (with the steady plate removed) would slide easily on its shaft and in others it would not....or could this just be loading of the cam pinion from one or both of the valves being open?
Thanks again for all your responses guys...much appreciated from this newbie.
 

Howard

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VOC Member
Much thanks to all for all the info, I have been reading everything I can lay my hands on...Richardson etc and was getting a little confused but I think I am on the right path now, I wanted to be sure of everything before I attempt to replace the Cam spindle.I am a little concerned though about the possibility of one (or more)of the gears running slightly eccentric because as I rotated the crankshaft I noticed that in some positions the cam pinion (with the steady plate removed) would slide easily on its shaft and in others it would not....or could this just be loading of the cam pinion from one or both of the valves being open?
Thanks again for all your responses guys...much appreciated from this newbie.

As you say, probably loads due to valve springs etc. but don't forget the spindle will be worn eccentrically due to the different loadings produced by the valves at different opening points.

I am curious how you intend to change the spindle with the engine in the frame, if I've understood correctly. It should be an interference fit requiring heating of the cases to remove/refit.

H
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi Howard, I intend to preheat the cases with hairdryers/fan heaters (beautiful Queensland sunshine?) etc for some time and then use a localised heat source such as gas torch and then use a CO2 spray on the spindle to shrink it some what. Then hopefully/carefully try removing with a slide hammer, feasible do you think?
 

Howard

Well Known and Active Forum Website User
VOC Member
Easy answer, I don't know. The only one I've done in situ was last weekend, and that fell out leaving me a different problem.

In theory it sounds ok, it should only be about 0.001" interference fit, I just worry about locally heating 60 year old castings, then applying shock loads. I'm sure plenty of readers have tried it, and will give their comments before too long.

Good luck.

H
 
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