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'49 Comet timing gears

kick69

Forum User
Non-VOC Member
#1
I'm a new VOC member and I have a '49 Comet, and it seems to have a timing gear noise. I have removed cover and found 2 aluminium gears (large & small idlers). Should they be replaced with steel ?, or what would recommend, our other comet runs alot quieter. Thank you for any suggestions, Jonathan
 

Len Matthews

Well Known and Active Forum User
VOC Member
#2
Comet Timing Gears.

It's Hobson's Choice nowadays. Current replacements are all in steel but don't expect a silent Timing Chest. Most Vincents clatter to some degree!
Len:) M.
 

Comet Rider

Well Known and Active Forum User
VOC Member
#3
Personal Preference

Hi Jonathon,

personal preference is to replace both with steel items. As Len say's you will find it difficult to get a perfectly quiet timing chest!
My view is that if you can hear it then at least you know things are still going round:D

Cheers
Neil
 

wmg73141

Well Known and Active Forum User
Non-VOC Member
#5
Noisy timing gears

Let us first assume that your gears have teeth in perfect order.
Secondly let us assume that the teeth have been cut on the correct pitch circle and that the tooth profile is also correct.
Third that all shafts and bores are within limits.

Given all the above the centre distance between the 24t half time pinion and the 79t idler should be 6.439” and that between the idler and the 48t camshaft gear is 7.939” allowing 0.002” for backlash. (Backlash on 16DP gears = 0.002” to 0.004”)

The shaft for the idler is moveable so the obvious thing is to make a pair of setting bars or something similar. Not everyone had access to a machine shop to make such items so the next best thing is to first set the idler in mesh with the half time and camshaft gear with ONE 0.002” feeler introduced between tooth faces of meshing teeth.
Follow this by the application of a thin layer of “marking blue” (or anything that will transfer a mark) on the tooth faces of one of the gears, the easiest is going to be the idler. By rotating the train some of the blue will be transferred to the teeth of the other gears and if meshing is correct the marks should appear across the centre of the tooth flank. Check the state of the steady plate for flatness and check the meshing of the gears again when it is all tightened up.

Straight cut gear teeth are never going to be entirely quiet due to a host of compromises in manufacture but the most important thing is that the drive is not transferred tooth to tooth completely smoothly which partly explains why helical gearing exists as in that case the load is always shared between two adjacent teeth.

Finally, if the existing gears are worn changing the centre distances, to reduce backlash for example, cannot improve matters so don’t bother!
 
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