Author Topic: Mk Main bearing housings  (Read 2137 times)

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Offline jeffen

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Mk Main bearing housings
« on: 08 Sep 2011 at 09:53 »
Does anyone know which face was used as the datum for the machining of the main bearing housings. I need to re-machine the front housing but I don't know which face to sit on the milling m/c.
Thanks
Jeff

Offline eddie

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Re: Mk Main bearing housings
« Reply #1 on: 08 Sep 2011 at 11:59 »
Jeff,
        Ideally, the crankshaft and the gearbox mainshaft should be perfectly aligned, so the datum face for machining would probably have been the joint face for the engine/gearbox. Presumably the crankcases would have been machined to the point that they could be bolted together as a pair, then held in a jig whilst the main bearing housings and cam shaft bores are line bored. At this point, the holes for the idlers would have been drilled/bored. With this in mind, it should be possible to establish the original position of the bore for the front bearing, working off of the 2 idler holes - obviously it will be in line with the holes and the distance from the idler holes can be worked out - but bear in mind that the 2 idlers are of different diameters.
      Having said this, early engines (Mk1 and 3) had one piece front main bearing bushes which seldom came loose (if correctly fitted) and even if they did, curiously, most of the wear occurred on the bush rather than in the crankcase itself. Later engines (Mk3 sports onwards) had 2 piece front main bearings which proved more troublesome with regard to working loose. However, being 2 piece, they always left an unworn 'rib' in the centre of the housing, making it possible to pick up on the original bore when remachining.
     When making the replacement bush, the bore (after fitting) should be 3 thou larger than the front journal - this was Mr Douglas's recommendation on the original Mk1 engines - any less clearance significantly increases the risk of the front bush picking up on the crank and turning in the crankcase (usually seriously damaging the flange of the bush on the locating peg).

      Hope this helps,
                                  Regards,
                                                 Eddie.

Offline jeffen

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Re: Mk Main bearing housings
« Reply #2 on: 08 Sep 2011 at 16:14 »
Once again Eddie you come to the rescue.  Your thinking is the same as mine, I just needed re-assurance.

Again thanks.
Jeff

 

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