Crankshaft alignment

Nourish

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Dec 2, 2009
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I've assembled the crankshaft into the cases on my 850 engine - fresh grind, new mains, new shells, new AN conrods, N.O.S Hepolite pistons(std), new liners fitted. I noticed that the T/S piston wasn't level with the top of the Barrel. The barrel head face is parallel to its base. I've put a drop clock on the head face of the barrel and and zero'd the clock on the outer side of the D/S piston, Moving the clock across to the inner side of the same piston I got -0.004", inner side of T/S piston -0.0015 and outside of this piston +0.007". So the D/S piston is tipping over 0.004" and the T/S 0.0085" - both tipping into the centre so I presume that the crankshaft cheeks aren't parallel - is this normal?
 
Did you go over the rods checking they were flat and the holes were in line and the same distance apart.
 
Nourish said:
I've assembled the crankshaft into the cases on my 850 engine - fresh grind, new mains, new shells, new AN conrods, N.O.S Hepolite pistons(std), new liners fitted. I noticed that the T/S piston wasn't level with the top of the Barrel. The barrel head face is parallel to its base. I've put a drop clock on the head face of the barrel and and zero'd the clock on the outer side of the D/S piston, Moving the clock across to the inner side of the same piston I got -0.004", inner side of T/S piston -0.0015 and outside of this piston +0.007". So the D/S piston is tipping over 0.004" and the T/S 0.0085" - both tipping into the centre so I presume that the crankshaft cheeks aren't parallel - is this normal?
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Not normal. I assume you had the crank apart for the grind, so it is possible that you didn't get the cheeks flat on the flywheel. If so, you have a "bent" crankshaft and need to "lick your calf over".

Idiots have been known to enlarge the top to holes so the studs would "go in". If that has been done, it's pretty easy to make a bent crank. When assembling the crank, everything must be hand tight and the cheeks must be solidly against the flywheel before tightening anything.

A much worse possible problem is that that person grinding the crank was not good at his job and ground them canted. Being canted .001" at the journal is probably enough to make the at least the .004" you're seeing!

The rods are easy to check - simply lay one on the other and the bottoms should be flat together and the small ends should just meet.

Long story short - take it back apart and figure out what's wrong!

Hint: Think of what a cow does right after giving birth and what if she doesn't do a good job the first time.
 
New parts are not guaranteed to be correct even from AN. You also have clearances in the main bearings, shell bearings, small ends which could be at play. The cheeks are a suspect but not the only one, if it is the cheeks then turn the engine so the pistons are at the bottom and the readings should reverse. If you have not got a DTI extension or a DTI depth gauge then use a normal digital caliper using the thin slide that comes out as the depth gauge.
 
have you remeasured by setting dial gauge on diffferent base positions to check consistency of the measurements?
 
I like Kommandos point about checking at BDC.

I would also urge much re-checking, turning over, and re-checking again.

I have set up tight squish on many Brit twins and have never had one that’s ‘right’ ie the exact same at each measuring point.

So IMO some variation is definitely normal. But 0.0085” is not.

So my advice would be to re-measure and see if the variation you have really is that bad.

If it is that bad, disassembly and inspection would be wise, but it’s distinctly possible that even if you do disassemble, you may find nothing obviously ‘wrong’.

At the end of the day you’re dealing with a 50+ year old bike that, even then, was from a different era regarding tolerances… and was made on worn out machines… by a less than ideally motivated workforce (Britain in the 70s FFS)…!
 
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Think of what a cow does right after giving birth and what if she doesn't do a good job the first time and Its a pre tensioned skipping rope - what are you lot on?

To be honest I think I'm more concerned that the pistons a tipping in than of being at different heights
 
Nourish said:
Its a pre tensioned skipping rope
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An accurate description of a Norton crank ref the pre superblend roller bearing fiasco is that at high revs it resembled a skipping rope and the main bearing roller edges were forced into the bearing track and destroyed the bearing. Hence the admittedly facious comment that yours is prebent ;)

Has your crank spent long periods at high revs.
 
I am finding hard to work out how a crank journal can be bent enough to cause an 8 thou drop to a piston top on one side in a fresh bore with a probable side clearance of 5 thou. Hence why the check when the pistons are at the bottom of the stroke is important in diagnosis.
 
kommando said:
I am finding hard to work out how a crank journal can be bent enough to cause an 8 thou drop to a piston top on one side in a fresh bore with a probable side clearance of 5 thou. Hence why the check when the pistons are at the bottom of the stroke is important in diagnosis.
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Trig used to be easy! Not 100% sure I've got it right!

Using round numbers. Let say the shells are .75" wide and the grind is tilted .001" (or crank bent). And let's say the piston is 3" across and that the distance to the top of the piston to the top of the journal is 6".

Then angle of the journal is .0764 degrees off from true. That makes the rod angle .0764 degrees off from true. That makes the top of the piston .0764 degrees off from true which equates to one side of the top of the piston .008" off from level but parallel to the journal.

So, a tiny error in the grind, a tiny error in assembling the crank, or a tiny bend in the crank can easily explain what he is seeing.

I agree 100% that measuring also at the bottom of the stroke is important. Also, after turning the engine by hand and then pulling the cylinders and inspecting the pistons and bore might say if the pistons are actually tilted. It's also possible that the pistons were made incorrectly - it's just odd that they would both tilt toward center; however, NOS is not always the best!
 
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marshg246 said:
Then angle of the journal is .0764 degrees off from true. That makes the rod angle .0764 degrees off from true. That makes the top of the piston .0764 degrees off from true which equates to one side of the top of the piston .008" off from level and parallel to the journal.
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But how much to the side is the piston moved, if it starts off in the centre of a 5 thou clearance it can only move 2.5 thou before hitting the barrel wall at the top and also at bottom of the piston.
 
kommando said:
But how much to the side is the piston moved, if it starts off in the centre of a 5 thou clearance it can only move 2.5 thou before hitting the barrel wall at the top and also at bottom of the piston.
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No doubt! Read my last paragraph! However, if the piston is 3" tall then the skirt bottom is .004" off so will fit. Remember these are rough numbers - I have not tried to use exact numbers, just answering you wondering how it could be.
 
OP should measure at BDC as you say Kommando.

I also suggest reversing the pistons and seeing if that does anything.
 
Fast Eddie said:
OP should measure at BDC as you say Kommando.

I also suggest reversing the pistons and seeing if that does anything.
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Also, with the cylinders removed and the pistons held parallel to each other, a straight edge across the piston tops inline with the pins. If it still shows off the cylinders are eliminated. Again, checked at TDC and BDC. In any case, if the same at TDC and BDC, it's most likely a crank grind issue. Even though improbable, it's possible it's the rods or pistons.
 
I was once told by Mick Hemmings that it was fairly common for Commando cylinders to be machined out of true and that this often lead, or at least contributed to ‘mysterious’ seizures.
 
OK, I've split the cases again and put the cranshaft up on V blocks via the main bearing inners. With the crank pins uppermost I've zero'd the DTI to the T/S crank pin outer side, then moved the clock across to the inner side and I'm getting a reading of -0.0001", across to the D/S inner I've got -0.0038" and the outer -0.0036".
Turning the crank over 90 deg using a vernier height guage in a balancing hole in the flywheel to keep it still. I measured again the height of the pins I zero'd on the T/S outer and measured +0.0003" on the inner side, -0.0054" D/S inner and -0.0050 on the D/S outer.
The journals were ground seperated from the flywheels and presume then that there is movement around the location dowel and the two locating studs?
Next step I suppose will be to strip the crank down again and investigate more!
 
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