S.R. engine rebuild conundrum

[Lord Flashheart]

Perhaps someone can throw some suggestions into the melting pot ?                                                                               I am re-assembling after a full engine strip, inspection and measurement,--new mains, sludge trap, everything checked out really well. Vibes had been a problem, so had the crank balanced.                                                                           Now, during reassembly, crank rotates and pistons glide perfectly with little resistance and no tight spots BUT ONLY OVER 3/4 OF A REVOLUTION. ---  From TDC I have rotation forward and backward but only by 120 degrees ( thereabouts) each way. The movement then comes to a positive and abrupt stop ( both ways) this equates to around 67mm downstroke.      The cases, barrels and crank assembly with pistons are in the frame, but no other items are attached. ( no primary or TS cases no timing gears, no oil pump. First thought was an over-long fastener penetrating the crankcases, but it appears not.    When trial fitting cases and crank following the reaming of the TS main, all felt good, but didn't approach BDC for fear of 'loosing' the pistons.                                                                                                                                       I'M TOTALLY FLUMOXED !

[limeyrob]

Check the big end nuts.  Some suppliers are sending B50 ones that look the same but are about 1/8" longer and just hit the case.  Look thought the sump plate hole.
This happened to me and it was the nuts, had to pull it all down and get bolts and nuts from another supplier.  Very frustrating.

[Lord Flashheart]

Hi LimeyRob,
That's a very good call, I did notice that the BE nuts were a little larger, and there's a good chance that you have it there. However, it was the abrupt and positive stop with no 'drag' that led me to wonder what else it could be.
There are witness marks inside the  lower TS case ( which I thought may be from a failed bolt in the dim and distant) these suggested that there is some eccentricity in the case(s).                                                                                     Time to maul the 'lump' out again ! ( and order another gasket set and bolts .Grr !)

[limeyrob]

The clearance between the nuts and the case seems to be about 1/8" but the nut is not rotating like the crank, its part rotation and part linear becuse the small end is traveling down with the piston.  Can't promise you its that but having had it happen it my best guess.
What I found was some suppliers selling B50 nuts and bolts, some selling AF nuts with BSC threads too. These don't fit well as the hex is bigger.  Some of the nuts are crimped, others split.  The longer bolts look fine with the nut but they catch on the case.  Did you fit new nuts and bolts?  If so can you put them alongside the old ones?

[Swarfcut]

    Abrupt stop? Something must be catching. So what's different in the way of new parts?

     Oval ground journals  or mis-aligned (bent) crank can give a tight spot, but something  preventing movement should be easy to spot. Piston skirt touching flywheel? Try without the  cylinder barrel, see if the fault is within the crank assembly.

  Splitting the cases should reveal all.  On an engine with primary case fitted, through bolts at the front of the chaincase which are too long can foul on the crank web. Reckon these bolt holes are clear in this instance, but worth a look.

 This old thread may help. See what "Shane" reckons about cases and cranks.

  https://www.britbike.com/forums/ubbthreads.php/topics/818508/a7-a10

 Swarfy.

[limeyrob]

Just to add as it could save time.  You can split the cases with the cam / gear still assembled. I can't remember whether you can leave the inner timing case / dynamo sprocket on, its saves time if you can.  But don't do what I did and forget the dynamo strap stud!

[Lord Flashheart]

Thanks guys for your responses.                                                                                                                                Limeyrob, re your second note. the first para, makes perfect sense, and the second I can concur with all your points.        Yes I did replace bolts & nuts, yes the nuts looked slightly longer, yes the hexes were bigger( AF), even ground a socket down a little to get down between nut and spot-face.  We live and learn, now for some CAREFUL purchasing.
Always assuming that opening up again confirms our suspicions.
Cheers
Flash