Crankshaft alignment

Hi,
I have just split my 850 crank to get in ready for a regrind and found that when I put the LH and RH cranks together after removing them from the flywheel and giving them a good clean, the bolt holes line up but the webs (where they bolt to the flywheel, sludge trap bits) are slightly offset to each other? Possibly a little bit of sloppy alignment when made. Also, some of the studs were a nice "tap to fit" where the others were a little sloppy. The dowel had worked itself out a little and pushed out one of the locking taps a bit so it had obviously been flexing a fair bit for it to creep out.

Anyway, all this has left me a little troubled but I have made two fitted bolts to go through the two halves near the journal, pushed in a turned brass dowel and popped bolts in the other holes and tightened it all up - I will be very surprised if ever runs true but I will give it to the shop tomorrow for them to check before grinding.

I am a bit concerned about bolting it all back to the fly wheel and I will use the two fitted bolts and dowel as my reference and then tighten it all up, no wonder these things vibrate, how do you ever get them to run true?

Any tips welcome

Supposedly it is the two bolts [studs] that are furthest from the center that are a snug fit and provide the alignment. The rest of the bolts are looser fit.

Actually the alignment is not always so good as they come from the factory.

I have been known to hold the journals in alignment using precision v-blocks and clamping the crank to a flat table and then reaming the holes for oversized studs. Jim

Hello Jim,

Thanks, I am a little more reassured! Here are some pictures. I actually made fitted studs which can be seen in the picture from over the journals. The other pictures simply show the offset. I have a good mind to get into action with my belt sander and make them nice and even but I suspect it is not necessary or really worth the effort?

Regards

As long as the journals are in alignment then the casting mismatch doesn't matter. Jim

Nigeldtr said:

Hello Jim,

Thanks, I am a little more reassured! Here are some pictures. I actually made fitted studs which can be seen in the picture from over the journals. The other pictures simply show the offset. I have a good mind to get into action with my belt sander and make them nice and even but I suspect it is not necessary or really worth the effort?

Regards
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IMO, whilst its apart and once the journals are reground, why not try and DIY "polish and balance" it.. That way you may you can spent a lot of time with the sander device and make an "ugly duck" into a swan........... Yes people are gunna say WTF, you cant do that at home in the shed... IMO,. Well yes you can... If you can balance a wheel in the shed you can do a twins crank ........ Unless of course your prepared to pay $500 and get the shop to do it!.. I think you can do it!!!!!!!!

As much as I hate to say this. If it were me working along in my shed trying to figure this out I would reassemble the cases with fly wheel, rods, pistons and barrels and I would be measuring the tops of the pistons in relation to the top of the cylinder to see if they match!

I do that kind of stuff because A.) I don't have enough experience to ease my suffering mind and B.) because it is the method that makes sense to me with my relatively primitive tools.

I am sure there are others here who can give you better advice on checking that alignment.

Russ

Hi,

Well I don't really know if it needs balancing, its going to vibrate anyway and as long as the isolastics do their bit I would have thought it will be OK. The crank will have to be checked before grinding but it looks like the journals are in the same place so should not be a problem. I was interested to hear from other members if the offset of the webs is standard or non-standard

you just gotta shake your head at the engineering thought process to build a crank like this???? Boggles the mind.

Wasn't there a story in one of the magazines where an ex-factory chap stated that when the cranks were divided, there was a bin for left and a bin for right halves and they were never matched up again ?

I had a Mk3 crank that had one journal .020" lower than the other (so a .040" difference in stroke). I gave it to Steve Maney to look at as he advertised a re-dowelling service at the time. I can't remember his exact words but I think that it invloved 'fookin' and 'boat-anchor'... :roll:

Isn't the important thing the relative positions of the big-end journal, the main bearing journal, the dowel alignment holes, and the bolt up faces? The shape of the "as cast" bits is what it is, by all means grind and polish the bits that seem rough, but I'll bet there are other things that you could waste your time on with greater benefit.
cheers
wakeup

I got to take this 750 crank apart today.
One of the crank bolts was extremely tight.
I just roughly sprayed the green/red paint to remember the alignment.
Nearly nothing in the sludge area inside.
Look how much weight is on the drive (left) side. No wonder there is flex at high RPMs.

Bob Z. said:

I got to take this 750 crank apart today.
One of the crank bolts was extremely tight.
I just roughly sprayed the green/red paint to remember the alignment.
Nearly nothing in the sludge area inside.
Look how much weight is on the drive (left) side. No wonder there is flex at high RPMs. ]

Click to expand...

And you wonder why the japs put their Morse primary and OHC drive chain in the middle :?:

Thats like a P3 Alfa . A Straight Eight , But actually Two Fours . So a Jap Four is Two Twins .

But the engine is more sensable with them at the ends , or more compact . If you look at that
sprocket like its got a tow rope on it , and ALL the Dynamic Accelerative Forces , & Decellerative ,
and osscilations between - skipping over bumps or ruts etc , THEN the Rods pounding away ,
theres a bit of torsion with lateral ( to bearing ) displacement in the Dynamic Forces going on .

Bob Z. said:

I got to take this 750 crank apart today.
One of the crank bolts was extremely tight.
I just roughly sprayed the green/red paint to remember the alignment.
Nearly nothing in the sludge area inside.
Look how much weight is on the drive (left) side. No wonder there is flex at high RPMs.

Click to expand...

I expect that if they'd put their minds to it, Nortons could have built a twin with a dynamo driven from the timing side and a single row primary...they could've called it a 'Dominator' It's a characteristic of Norton over the years that most of the 'improvements' brought a new set of problems. We do still seem to like 'em though

The little mis match at flywheel cheeks is a non issue as not very far from center of rotation to have much influence. Isolastic can not isolate horizontal imbalance very well so attention to equal mass on both sides is worth while. With the iso's of a Commando, likely a V block DIY crank balancing like Jim Schmidt outlines in his Race Manual should work out nice. I could not resist some casting clean up when parts in hand and a bench grinder handy, but short of polishing all over its just for cosmetic pleasure and pictures.