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Late Navigator Gearbox blues!


Dear all,

It is fascinating how a Lightweight can always find new ways to challenge and frustrate you no matter how often you have rebuilt one!   Having had a highly reputable precision engineering company replace my late gearbox bronze bushes and mainshaft bearings with new items from the NOC shop we discovered that the lay shaft was too tight a fit in the crank case end pegged bush.  Having searched high and low for a suitable hand reamer that could ream the 'blind' bush and failed I had them ease the lay shaft by 1 thou" so it was a play free fit.   

With the correctly assembled gearcluster in place everything runs freely but when I fit the off side gearbox end plate the gearbox tightens up so much the sleeve gear is difficult to turn.   The main shaft rotates freely and the kickstart operates it without any binding so I am pretty certain that it is the lay shaft that is running tight in the crankcase end bush.  I have checked the lay shaft fit in the kickstart end new bronze bush and it runs very freely.  I can only assume that the new crankcase bush 'shrank' a few thou when it was pressed in and my options are to ream it out a few thou or skim the lay shaft a similar amount.        

Has anyone experienced this problem before and if so how did you solve it?    Is there anything else I might have missed in assembling the gearbox?   I have read my original Maintenance manual and the later publication and am pretty certain I am assembling it correctly.

Standing by for words of collective wisdom!





Hello Nick,

I had a problem like this. Eventually I found it was of my own making.

Are you reasonably sure the gear selector is in neutral position when securing the gearbox end plate into position. You can get gear dogs hitting gear dogs if one of the gears is selected. Best fix for this is to rotate the sleeve gear as you tighten the gearbox end plate.

You have tried the layshaft into the crankcase bush and it rotates freely? They do shrink fractionally when pressed into the hole. 

I doubt the above is your problem.

I solved my layshaft problem by just putting 1st and 4th gears on the layshaft. Re-assemble rest of gear cluster and insert. Now turn the sleeve gear as you tighten the gearbox end plate.

If the sleeve gear gets difficult or impossible to turn, the layshaft endfloat is actually a nip. 

In my case a self made replacement kick start shaft bush had too thick a shoulder. Thinning it restored endfloat.

The layshaft is held between the shoulder of the crankcase bush and the shoulder of the kickstart bush. If either bush is not fully home, or the shoulders are too thick, the layshaft can get nipped up.

Be cautious.


Sage advice as always Peter, I will find some time later today to go and assemble the gearbox as you suggest and see if the new layshaft crankcase bush is the cause, I haven't replaced the kickstart bush so it can't be that end that is responsible.   If the crankcase bush is the problem can you or anyone else suggest a tool or practical way of 'easing' it by a few thou to restore the end float?   Having meticulously assembled the rest of the engine with new bearings and gaskets at considerible expense I am loath to strip it all down again!!!!    Would an alternative be to ease the kickstart bush which is easily accessible?  

Don't you love Lightweights!!!   Thanks for all the help.  Nic



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