850 refit barrels after splitting cases

Yes, the advise is correct. 

Lightly fasten the crankcases together and the check to see if the top flat area where the barrels fit, is perfectly flat without one side being higher than the other.  If all is good, then fit the barrels - you don't need a gasket underneath for this - and nip them down with the nuts.  You can then tighten up the crankcases and then remove the barrels and you will be left with a perfect mating surface.

I have always found, both on Commando and Atlas, that as long as the crankcase flange dowels fitted and tight, the two casing halves align perfectly. Easily checked with a steel rule across the cylinder flange.  Not convinced that the two crankcase mouth studs, particularly the 1/4” stud, would hold any misalignment corrected by dry fitting and tightening the cylinders?  Just my opinion of course.

Hi Paul,

I would like to believe that there being only one dowel the cause of your issue? Like Andrew says, for my engine, 2 dowels in and my cases match up perfectly. The dowels also ensure that the main bearings and camshaft bushes are in line. Have you tried a second  dowel?Are your cases a matching pair? (i.e. same machine shop numbers). Something isn't right if you are only able to have one dowel. 

The dowels (06.2621) are only around £4 each, so treat yourself to a second one and do the job properly. 

Regards,

George 

there was a case set without the sump filter, which is now thought of as a backward step. Also had oil pickup problems at sustained high revs. Later models reverted to big sump Drain/filter.

The 1972 & '73 750s had a breather low down at the rear of the cases. It included a foam 'Breather separator' in a vain attempt to prevent the dreadful wet-sumping design from returning more oil via the breather on the rear of the cases than the return gears on the oil pump which mostly ran on air...It can and should be done away with, but this brings with it the need to grind the cases to enable the oil pick up to occur before the level rises too high.

Prior to 1972, and later all of the 850s there was a substantial gauze strainer in the sump which prevented large lumps of metal, whether magnetic or not from passing through the oil pump gears. This can only be a good thing...but for the last year and a half of 750 production, it was absent which, despite the strongest 750 crankcases, probably made them the worst of the Commando engines.

The mods to ypur case (you can see where the weir has been milled away) are necessary in order to prevent wet-sumping at high revs. It often went un-noticed at the time as much of the oil from the sump was being blown up the breather...If that was moved to the 850 position (as yours has been) then there were problems.