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Crank runout

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi guys,
Fix one thing find another!!! Still haven't allowed the Comet to fire a shot in anger yet, however finally finished the timing case and thought i would have another peek around the Primary chain side before I closed her all up.
When rotating the crank I noticed the chain go tight then loose then tight ...... Hugo gave me some good advice to check for a knackered chain (about to do that) but prior to the advice I removed the ESA and clutch to check for runout on gearbox mainshaft and crank shaft.
Depending on how I do the measurement I am getting approx .003" to .005" runout on the end of the crank? what is the general consensus of this value?

Regards
Kevin
 

nkt267

Well Known and Active Forum Website User
VOC Member
One thing that can happen with Comets,especially if they have been coupled to a sidecar, the gearbox mainshaft can bend.The one I had was 50thou out of true and that made the chain go tight and slack when the engine was turned over. John
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Hi Kevin. John makes a good point and there are a number of possible other causes for the primary chain going tight, such as eccentric/worn sprockets. Just a comment on the crank -at the end of the day, provided the crank runs relatively true where the main bearings are located, any mainshaft run-out further from the bearings is not overly important. I'm unsure how you've measured the run out as I'd have thought only the splines would be exposed with all the ESA parts removed. Now if you've left some of these parts on, such as the thin-wall parallel ESA sleeve ( I can't quote part numbers ) who is to say this was machined/ground concentric? Provided there are no nasty noises and the engine runs smoothly it pays not to go looking for trouble. Cheers David
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
One thing that can happen with Comets,especially if they have been coupled to a sidecar, the gearbox mainshaft can bend.The one I had was 50thou out of true and that made the chain go tight and slack when the engine was turned over. John
Thanks John, for all I know about this Comet it may have been coupled to my ex wife in a previous incarnation, some people get melons, I got a mullet :)
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi Kevin. John makes a good point and there are a number of possible other causes for the primary chain going tight, such as eccentric/worn sprockets. Just a comment on the crank -at the end of the day, provided the crank runs relatively true where the main bearings are located, any mainshaft run-out further from the bearings is not overly important. I'm unsure how you've measured the run out as I'd have thought only the splines would be exposed with all the ESA parts removed. Now if you've left some of these parts on, such as the thin-wall parallel ESA sleeve ( I can't quote part numbers ) who is to say this was machined/ground concentric? Provided there are no nasty noises and the engine runs smoothly it pays not to go looking for trouble. Cheers David
Thanks David, you are spot on, I used the ESA sleeve but held it in one place as I rotated the crank, the only other issue in that respect would be thatany burrs/corrosion etc on the crank would give erroneous readings.
Since I have never yet had the opportunity to start this little ferret, I can't tell if there are any nasties till I do...........some day......hopefully...
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
If I had that much runout, I'd would want to know why before going further with the assembling of the primary drive. Flywheels, Twin and Single left the Works with .0005" runout and there is one man in Australia at least who can achieve this!
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
Aye Trevor, you're right but the Workshop Instructions still say .001" runout on a 6" mandrel. (Though where I'd get a baboon that small I'm not sure.) My point is; it could be something simple and easily fixed or it could be evidence of mechanical malfeasance on an operatic scale.Since the owner plans to use the machine to its full capabilities, best not to be left wondering. Cheers, F5AB/2A/7945 since1970
 
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