E: Engine Main bearings

Bill Thomas

Well Known and Active Forum User
VOC Member
In does not matter, But as per Greg's photo, The steel cage is bigger one side and might get damaged
Trying to pull it off, The old brass cage type was the same both sides.
Try Greg's way of heating the bearing with a hot air gun or hair dryer ?, Sounds good to me, but I have not tried it.
 

greg brillus

Well Known and Active Forum User
VOC Member
Learn't to use a heat gun all the time when I worked in aviation, as many bolts and pins are a very neat fit into holes in alloy brackets or similar and you can't use a naked flame. The heat guns will get things very hot if you apply it for a length of time, but the bearings come off without too much trouble, even though the shaft absorbs some of this heat. There are many ways of doing things, but some are just easier and much faster.
 

Tony Wilkinson

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Non-VOC Member
Bill, yes I thought the oil scroll was going to fix the problem. As I said above, my engine actually lacked this part. But it's made no difference. I appreciate that I need to centralise the rods, but there seemed little point in moving on to this before I got the crank turning without banging into something! I have had a mail conversation with Davidd on here and he suggests a cheap bearing seperator, ground to fit, to start the bearing off the shaft, so that is my next purchase. My thinking now is that since I probably need to shim the bearing to get clearance anyway, I am going to have to go with whatever that takes and hope the rods line up. I have to get the drive side free turning, as that is the side that is tightened up against pulling the crank over, not the timing side. I don't like the fitting on the main much myself, it is horrible, but I'm stuck with it unless I either scrap the case for a modern replacement or undertake major machining work that I can't do myself. I do have some F1 contacts and an engineer friend who told me that TZ750 Yamahas were raced in the Island with the bearings glued in with Loctite. I dont intend to use the bike THAT hard! In the meantime here are some photos of the crank. I am starting to wonder where this build is going as every time I fix one problem another crops up; it's beginning to get me down if the truth be told...
It is true that main bearings in TZ Yamahas were (are) often put in using Loctite to stop them spinning in the cases. I used to do it with mine as well. The difference being, TZ's are revved to well over 10000 RPM.
 

oexing

Well Known and Active Forum User
VOC Member
Actually the bigger difference is the horizontally split engine case - no ? In that case you cannot easily have a good shrink fit when assembling the engine so in consequence you see walking races after some higher mileages. Unless there are features preventing races to spin like keys or pins so no spinning is possible.
Good engineering really cares for light shrink fit gripping races well enough till a bit higher than operating temperatures. I go for sizes that free races at about 120 degrees C so no big problems with spinning races and no factor in internal play in the bearings when hot.

Vic
 
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