problem reassembling 1928 Sloper engine

[Paul.W]

I hope someone can advise/help;

I have a 1928 350cc OHV Sloper.  The engine was very noisy, so I removed the barrel and found lots of big-end play.  I dismantled the engine and had the bottom end rebuild by Alpha. I picked it up last week and decided the best way to escape the festive celebrations for an hour or two was to start the rebuild.  However......

When I assembled the two halves of the crankcase around the flywheels/bottom end and tightened up all the bolts I ended up with the crank locked solid.  Loosen off the bolts a turn or two and all becomes free.  It would appear that the crankcase halves are "nipping up" the crank when the bolts are done up tight.  It is as if the newly rebuild crank and flywheels are slightly wider than originally.  I can't see any way of adjusting or compensating for this (other than putting the entire crank assembly in a lathe and facing one of the flanges that press against the bearing in the crankcase half - which can't be the correct course of action).

Can someone tell me what I'm missing, or have Alpha re-assembled it oversize (which I find hard to believe - they've done enough of them).

As an aside, I have a spare assembled crank and flywheels (I couldn't simply swap, they are slightly different).  If I measure across the flywheel faces the dimension on the rebuilt one is about 10 thou more than on the old one.  That may not be relevant as its the distance between the flywheel flanges that matter and I couldn't measure that, but 10 thou might be enough to cause it to jam up.

Any advice/comments gratefully received.

[JulianS]

This service sheet has some info on Sloper crankshafts.

[Dean Southall]

Other than thinking you are simply experiencing orignal build tolerance issues(  ie. did they originally have to be individually fettled?) my first port of call would be to put the question to Alpha. As you say, they have a wealth of experience and they may come back with an explanation like you posed. On the other hand, even the most experienced of us can have a bad day.
Good luck, I hope you get it sorted soon and with minimum hassle.

[ducati2242]

Id be surprised if Alfa had built it incorrect . Not knowing anything about your engine but my thoughts would be with the main bearings . You havent mentioned changing them but I am surmising you have . It sounds like they are either slightly wider or havent gone into the cases far enough . Could be wrong though as I know nothing of your engine . Most Engines I have built need 2 to 4 thou end float when fully tightened up . But I havent had anything earlier than 1950 .

[Paul.W]

I think JulianS may have identified the problem with that service sheet.  The dimensions were altered at some point by 1/16", so I suspect Alpha have assembled it to the wider dimension. Unfortunately they are out until the new year, so I can't talk to them until then so my plan to spend the holidays assembling it may have to be revised. 

The service sheet does say that if a later crank is fitted to early crankcases 1/16" must be machined off one face of shaft boss.

[Paul.W]

Update......

I have most of a spare engine, so I've been comparing fits.  The old crank and flywheels are a prefect fit in the crankcase off the bike, so it looks like Alpha have (dare I suggest it) assembled the crank with a crank pin of the wrong length. A solution might be to take 1/16" off the crank boss as described in the service sheet - but I'll discuss it with Alpha first and see what they say.

[Paul.W]

All is clear. I took the crank back to Alpha, this time along with the crankcases.  They identified that the problem was that the crank nuts were very slightly  deeper than the originals, and one of them was just fouling the internal crankcase flange as it was tightened up.  Had the nut been in a different orientation it would have not touched, it was the very tip of the hex that was touching.  They relieved the top of the nut very slightly and all is now well. 

All I have to do now is try to remember how to put it all back together.