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Primary Gear Clearance Check


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#1 unburntfuelinthemorning

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Posted 17 May 2019 - 07:37 PM

I've been reading so many primary gear threads it's making my head spin. 

 

What is the factory recommended crankshaft to bush clearance range for a 998 primary gear with a Deva bush? 



#2 Moke Spider

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Posted 17 May 2019 - 08:44 PM

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#3 unburntfuelinthemorning

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Posted 17 May 2019 - 09:49 PM

Thanks.  So I take it the crankshaft tail should be 1.500" exactly giving a 0.003" to 0.004" clearance when the bush is machined as above.  What is the maximum allowable clearance?



#4 Moke Spider

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Posted 17 May 2019 - 09:55 PM

Thanks.  So I take it the crankshaft tail should be 1.500" exactly giving a 0.003" to 0.004" clearance when the bush is machined as above.  What is the maximum allowable clearance?

 

0.0045"



#5 unburntfuelinthemorning

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Posted 18 May 2019 - 06:27 AM

That's not much wear then.  I hope one of the two primary gears I have will be alright as I don't want to use a non-Deva one. 



#6 unburntfuelinthemorning

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Posted 18 May 2019 - 11:45 AM

I make the internal diameter of the primary gear from the better of the two engines I have 1.5045" but the crank tale measures 1.499".



#7 RooBoonix

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Posted 18 May 2019 - 02:29 PM

Personally, if it was a choice between a Deva bushed primary gear with a thou too much clearance, or having to use a non-deva primary gear... I'd choose the Deva one.

Moke is the man though, I'm interested to hear his view

#8 Moke Spider

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Posted 18 May 2019 - 07:55 PM

Personally, if it was a choice between a Deva bushed primary gear with a thou too much clearance, or having to use a non-deva primary gear... I'd choose the Deva one.

Moke is the man though, I'm interested to hear his view

 

The official figure is as I've posted above and that is really what I need to be saying in these pages.

 

However, I have pulled some very high mileage gears over the years, one had 0.009" clearance and there was no issues that I could Identify from this much clearance, other than it will wear faster and faster as the bearing contact surface area is getting smaller.

 

Eventually though, they will get to a point where they'll leak oil and / or crack the centre out of the clutch plate, but until they get real bad, they won't damage the crankshaft, but usually there'd be other issues apparent long before they get to this point.

 

In my books, there is only one type of material to use here.



#9 unburntfuelinthemorning

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Posted 18 May 2019 - 08:55 PM

It seems some people have had success with the new types of primary gear bushes but I think I'd rather try to get a new old stock item as there weren't exactly many problems with the factory ones once they used Deva bushes.

 

I suppose the only way to increase its service life is to not hold the clutch down unnecessarily.



#10 unburntfuelinthemorning

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Posted 23 May 2019 - 03:32 PM

I've got a new old stock primary gear so that's one less problem.

 

Should I be replacing the drop gears as a set though or is that only necessary if the other gears are knackered?



#11 Moke Spider

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Posted 23 May 2019 - 06:34 PM

While it would be best practice to replace them as a set, I only replace what's worn.






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