Crank is stiff to turn

you don't give any details about what you have replaced, but my first thought is that the pistons may be wrong for the crankshaft and are touching the flywheels at the bottom of the stroke. 500 cranks are full circle flywheels 600 ,650 and 750 all have flat areas to allow clearance for the bottom of the pistons skirts.

Was it free before the barrels went on?  

Maybe one of the faces (crank or barrel) is not flat.  Do the case halves match?

If you remove the barrel, maybe try a dial gauge on the flywheel to check it it is running true. 

If you had the conrods off and fitted new shells,did you check the rods were free to turn over a complete revolution? ,or just rocked them?.  Its not unknown for old rods to go a little oval and the act of fitting new shells to make a tight spot. Also check the cam followers are all free .Cylinders not parallel? I hope not!

Sometimes when fitting new main bearings the cranks can appear stiff especially if the cases haven't quite gone back to their seated position.  I don't know your age Robert but here's a little trick if you're strong enough.  Place a thick piece of hardwood  on a hard surface (concrete or paving flagstone is ideal)  lift the engine above the wood such that the drive-side end of the crankshaft is about 9 inches above the wood - then carefully release your grip a little and let gravity do its work.  You can retain enough grip to stop it falling once it hits the wood.  A couple of hard shocks usually lets everything slot back together.  It is usually the timing-side crankcase half that hasn't quite settled.  The engine nuts will need a tweak afterwards.  Good luck, Howard 

I would put poorly ground crankpins top of my list with distorted rod eyes or caps as a close second.  Alternatively the corners of the crankpins may have the wrong size radius and be snagging the sides of the big-end shells.

I would also be looking very closely at the rebores. Norton twin often had barrels manufactured with cylinders that were not parallel. Generally this shows up after a rebore when you find one side of the barrel has more metal thickness than the other.  I don't think it is the pistons touching the flywheel. They either foul big-time or not at all if decent pistons are fitted..

The first time I fitted new pistons to my 650SS, they  jammed on the flywheel. I had to take them off and relieve the piston skirts to get clearance. I'm not sure if 600 pistons, although the same bore, have longer skirts. Maybe that's the root of the problem?

If the pistons were for a 99 then the skirts would be too long for a 650.  But it would just go CLUNK  and stop I think.  When I rebuilt the 99  with new shells on a well used crank I had to re-fit the rods a few times to get free movement.Did the pins pass freely through the small ends? .

hello for a start of the barrels do taper out to the bottom of the barrels if the rebore as not allowed for it that where the pistons are fouling  and the bore will need correcting  I had this sometime back When I then owned And Wartburg Tourist 353 mk1 and I had the block rebored the machinist did not tapper out at the bottom of the stroke  And could not even get the pistons in So I took it back to the engineering workshop  in Selby  and explained to them the problem  So they rigged a machine up to do the tapper and then I could fit the pistons OK  So you may find your self going back to the engineering shop  to sort this out,     Yours  Anna J

Don't ask questions ,you'll only become more confused,  because it's obvious to me that some of the answers aren't relevant to the question. Do something positive , very carefully remove the barrel and start from there , otherwise you're just going to be guessing.

That's really odd.  The piston skirts should not get near the case .  I'm wondering if you have mixture of parts. What are the case numbers. The pistons do not look too damaged in the important areas as long as no cracks.Some photos of the engine may help. A wild guess, 650 /750crank in 88/99 cases and 99 pistons?. A conumdrum.

The issue seems to be the piston skirts, have they been marked by the rods on the inside of skirts?. Marks on the rods indicate that the rods may have been pushing the pistons against the cases.  Supplier should change these as wrong for the motor. If no luck there I would alter the skirts to match the old pistons.

I can see horizontal marking on the rod faces. Could be the pointer.

Very interesting thread so far.  Robert you say that you had the crank reground and new big-end shells presumably ?  Did you take the crankshaft apart or did the workshop, doing the regrind,  do that ?  Sometimes the three segments can be buggers to fit back together properly.  My thoughts are that the two pistons are incorrectly aligned with the bores in the barrel causing the pistons to scue against the cases at the bottom of the stroke.  Another question please Robert - I presume you fitted new main bearings when you rebuilt the bottom half ?  Did you use the original lipped roller on the drive-side (MRJA30) and a rigid ball bearing on the timing-side?  When you tightened the worm nut to pull the crank against the t/s bearing you did remember to fit the triangular washer and the tin bearing cover.   If you have fitted Commando roller bearings both sides there may be an issue with the end-float.  Cheers for now, Howard    

You had the right answer in your 2nd post . Piston skirts too long for the longer stroke of this engine, At some part of the stroke maybe the rods are coming into contact with the inside edge of the skirt and rocking the piston by just enough to  catch the .case ,or they are long enough to catch anyway. At one time Mix and match was done to the 88/99 motors with interesting results .Not seen your problem before though. Very entertaining!.

I have some 99 pistons ,I will compare them to your photos.  My 99 runs with 650 SS pistons   So I have been aware of the differences.I think your original pistons were wrong too but had been made to fit. Now an extra +10  is making a problem.

It sounds as if your cases had a  interesting time when the cam shaft was damage. I wonder if the cases are distorted somehow? When you remove the pistons and before you split the cases, maybe you could check if the flywheel runs true. No point in stripping it again if it is true, is there?

Looking closely at your photos ,it appears that the distance from c/l of pin to bottom of skirt is greater on the new pistons. That does not in itself explain the height of the scrape on the skirt. However I think the rod face IS making a contact with the inside edge of the skirt and forcing the piston over into the case . Clean up the pistons and shorten the skirt  and you may find disaster averted. You should get your money back or correct replacements but I won't hold my breath.

The overlap looks like half an inch. I don't see how pistons could do that. But bent con rods might. That's my theory....

Seems reply is not working at the moment. 

The bottom spigots should go into the crank case. So it should not be possible for pistons to touch cases..unless barrel is back to front!

A photo of the inside back face  bottom edge of the pistons may help. David, if 99 long skirt pistons are fitted then they will go further down than the barrels and hit the  casting  flash in the cases. If you are right I will buy you a beer next time we meet.!.   

The overlap looks to be very small, doesn't it? I see it was a Plumstead bike like mine (letter P), and my PO who put mine together said the fit up left a lot to be desired! Maybe it's simply that the case castings were never finished off correctly, and the extra few thou on the piston diameter was the last straw.  So trim the hole with a burr in a Dremmel?

I have a Dremel and it has been a revelation  ,doing all sorts of tasks I would not have expected.  What I have not found is a blade that will cut alloy without smearing . Something like a tct blade in minature needed. The piston issue may be just down to AMC  sloppy  finishing ,not clearing casting excess .    That's what you get with a "FAKE" Norton !!.

With these old motors even the really old hands expect to find a few surprises , wrong parts made to work ,new parts that are poorly made to indifferent patterns , missing parts that are not clearly shown on the literature,anything is possible . don't beat yourself up, most of the satisfaction is in getting the thing to run reliably and efficiently. Something I have learnt is don't try to renew original parts till you are sure you have better replacements, and don't throw anything away!.

All pistons are tapered with the bottom largest as the heat from the combustion chamber makes the top of the piston expand more at running temps. The shine on the bearings is where they have contacted the crank journals, did you lubricate the shells with assembly lubricant before torquing up the con rod bolts.

I have no doubt that the rods are slightly oval, Something that happens over time. The Ideal would be new rods. And they are availiable,if a little expensive.  The next best thing would be to get the rods re-sized,but as you say,not practical at present. You could give them a thorough clean ,and the back of the shells to make sure there is nothing there. I am guessing the shine is near the ends of the shells where they meet?. In the old days they would have "scraped" the bearing to releave the tight spots, I don't think you can do that with the shell facing you have.If it were mine I would scrape the rods (not shells) at the tight areas a thou or so till it all turns freely.Also give the crank journals a polish with very fine carburundom paper. All at your own risk. I have done this before and no problems but its usually frowned upon!.You could get a plastigage Kit thro the post to check bearing/journal clearance.Also check the big end caps not switched.

An interesting thread, I assume the early barrel that you have used is spigoted did your old barrel have spigots? I have had a similar problem with my '67 650SS which the previous owner fitted with  a barrel from a Manxman [spigoted] I had the engine rebuilt with GPM pistons and found in my case the skirt touched the flywheel but the piston also touched the head both problems were cured by a bit of light machining. I also believe the pushrod length was shortened when the spigot was removed.

Those  shell photos are exactly what I expected to see.  Running too tight , no clearance for the necessary oil boundary.