E: Engine magneto bearings

erik

erik

Well Known and Active Forum User
VOC Member
on my comet the rear bearing ,the one Close to the ATD is loose.there are three of the paper spacers mounted,but i think the hole in the Body is to wide.how should this repaired?making the space bigger and Fitting a nonmetallic spacer?for example delrin or pertinax?otherwise the mag is working good.regards Erik
 
Magnetoman

Magnetoman

Well Known and Active Forum User
VOC Member
It's difficult to be sure how to approach this without inspecting the damage and measuring it, but if the three paper insulators you have are standard Lucas items it means the hole is at least 0.030-0.040" larger than the OD of the bearing. This means that with one standard 0.010" paper insulator you would have an excess clearance of at least 0.020-0.030".

I'm jet lagged and not near my data, but let's say the OD of your bearing is 35mm. Given that diameter, machining a thin-walled shim of the required wall thickness from brass would be pretty easy. Again, assuming close inspection turned up no other issues, the general procedure would be to start with a larger dia. brass rod (e.g. 1.5") and bore a hole in it of the proper diameter (i.e. the OD of bearing +0.010"). Then turn the OD down to whatever the current ID is of the worn housing (plus ~0.001"), part it to the required length, and press it into the housing.

It's likely the hole in the housing is oval so carefully centering it and taking a few thou. off to make it round again would be done before making the shim to size to fit the hole.
 
erik

erik

Well Known and Active Forum User
VOC Member
why making no bigger or thicker spacer.let`s say 2mm thick.there should be enough material?
 
Magnetoman

Magnetoman

Well Known and Active Forum User
VOC Member
The reason not to make a thicker spacer from an insulator like nylon or delrin is the thicker the material, the more it can distort. If there's room you certainly could bore the hole larger and fit a thicker spacer of something like brass that won't distort, along with a paper insulator, but I wouldn't use any kind of plastic. If there was enough radial thrust to enlarge the Al hole there's enough to distort something softer than metal.
 
Magnetoman

Magnetoman

Well Known and Active Forum User
VOC Member
Young's Modulus measures how deformable a material is, i.e. how much it distorts per unit force applied to it. Although PEEK is quite good for a plastic (~50% better than acetyl), its Young's Modulus is ~25x lower than that of brass. That is, the same force applied to a PEEK insert would result in 25x more compression of it than one made of brass. This might or might not be an issue in this particular application but, if it were me, I would use brass.

I only mention brass because free machining versions of it are much easier to work with than most other metals, which is a help when dealing with thin walls and relatively high tolerances.
 
Top