Re: Gearbox Sludge
Posted: Sun Oct 17, 2010 10:01 pm
It might be wise to also point out that varying the thickness of a paper gasket over the factory stock item might have unintended consequences in extremis..........
In particular thinner or thicker gasket paper between the gearbox front cap and the gearbox body will affect the meshing of the crown and pinion wheel............ (BS adjusted the mesh here by selecting from various thickness washers for part # S1383 , and 1383. I believe the std thickness range was 83, 93 and 102 thou; vernier accuracy, measured in Maidenhead.
So varying the gasket thickness by 10 thou is the equivalent of changing to a different washer...... And there was only 30 thou of adjustment range in the factory....... to put that in context quality newspaper is about 0.1mm or 4 thou.
The block to crankcase gasket is primarily there to provide a thermal break to help keep the crankcase cool and prevent premature heating and expansion of the inlet gases. You'd be very unlucky to clout a piston if you left it out and will likely not notice the compression drop in running performance if you doubled the thickness! (Then again if somebody has been radically skimming the block as per recent posts then it is worth checking carefully....
)
Waterpump to gearbox housings are not critical.........within reason.
My personal reasoning for Centuries and later 40+ models having thick composite packing gaskets here is that the plastic impeller is only a friction fit on the drive shaft on these models and can slip downwards until enough rust forms on the drive shaft to fix it solid! During this time the impeller gently sacrificially wears away the top surface of the thick gasket until the impeller freezes solid on the shaft......... Without the thick fibrous gasket the gearbox housing would quickly wear away the softer plastic impeller, if it was in contact.
Earlier metal impeller models are secured with a steel pin and get away with thin cardboard since the impeller height is fixed from the start.
Peter
In particular thinner or thicker gasket paper between the gearbox front cap and the gearbox body will affect the meshing of the crown and pinion wheel............ (BS adjusted the mesh here by selecting from various thickness washers for part # S1383 , and 1383. I believe the std thickness range was 83, 93 and 102 thou; vernier accuracy, measured in Maidenhead.
So varying the gasket thickness by 10 thou is the equivalent of changing to a different washer...... And there was only 30 thou of adjustment range in the factory....... to put that in context quality newspaper is about 0.1mm or 4 thou.
The block to crankcase gasket is primarily there to provide a thermal break to help keep the crankcase cool and prevent premature heating and expansion of the inlet gases. You'd be very unlucky to clout a piston if you left it out and will likely not notice the compression drop in running performance if you doubled the thickness! (Then again if somebody has been radically skimming the block as per recent posts then it is worth checking carefully....
)Waterpump to gearbox housings are not critical.........within reason.
My personal reasoning for Centuries and later 40+ models having thick composite packing gaskets here is that the plastic impeller is only a friction fit on the drive shaft on these models and can slip downwards until enough rust forms on the drive shaft to fix it solid! During this time the impeller gently sacrificially wears away the top surface of the thick gasket until the impeller freezes solid on the shaft......... Without the thick fibrous gasket the gearbox housing would quickly wear away the softer plastic impeller, if it was in contact.
Earlier metal impeller models are secured with a steel pin and get away with thin cardboard since the impeller height is fixed from the start.
Peter
