Main bearing alignment?

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Avantone
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Mon Jun 29, 2020 8:53 pm

Anyone check that their mains are truly aligned before doing a final fit on the crank?
If you do, how? Bar with bearing ID?
Thanks


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Tony

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Warkton Tornado No.1
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Tue Jun 30, 2020 2:48 am

I've never had to check, but I understand that if alignment was likely to be out, due to extensive welding, then axial alignment would need it.

Unless I pulled in a favour with somebody that works in a machine shop with XYZ inspection facilities, I suppose the crankcase/magneto flange assembly could be cross carriage mounted on a lathe with a centres mounted bar to which a DTI could be attached.....

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Avantone
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Tue Jun 30, 2020 11:25 am

Warkton Tornado No.1 wrote:
Tue Jun 30, 2020 2:48 am
I've never had to check, but I understand that if alignment was likely to be out, due to extensive welding, then axial alignment would need it.

Unless I pulled in a favour with somebody that works in a machine shop with XYZ inspection facilities, I suppose the crankcase/magneto flange assembly could be cross carriage mounted on a lathe with a centres mounted bar to which a DTI could be attached.....
I've never checked either, however that's not to say a misalignment isn't there.
The potential for error is assuming the bearing centres are aligned, and bearing faces parallel where you have an element variability in the mag housing gasket faces (perpendicular to the bearing?), how it tightens down, not to mention a mix and match of parts.
If there is a small misalignment, when the mag housing is tightened down, it would be taken up by slack in the bearings and potentially some crank flex. You'd potentially see a stiff crank on the mains that may be put down to "new components" and new oil seals until they wear in.
Just a thought, but a pretty critical component, so something I'll check now going forward for peace of mind.
Tony

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Warkton Tornado No.1
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Tue Jun 30, 2020 12:12 pm

As the subject of crankshaft flex has now been mentioned, my belief is that it is something not fully understood. Probably by me especially!

I once built a race engine in which the drive side deep groove ball bearing was replaced by an NU type roller bearing of the same dimensions, but was in three parts to use the side of the rollers as location axially. My belief is that it stiffened the whole assembly but, in doing so, restricted the crankshaft flex.

The engine was turbine smooth up to about mid revs, but then there would be a kind of thrumming which I believe restricted the revs.

Upon strip-down, you could see that the magneto side NU bearing was taking the brunt of the change in dynamics.

Fast forward several years & I can tell you that in some race engines, not only are the inner magneto seals omitted to lower primary compression, decrease drag & increase the speed capability, but the bearing fitted magneto side might well be........a magneto bearing, as in a (often) separable ball bearing once popular in motorcycles.

That arrangement must allow for even greater flex.

A quick look @ alternative bearings showed up a self aligning, double row ball bearing 22205E that has ample speed & load ratings :idea:

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Avantone
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Tue Jun 30, 2020 12:51 pm

By flex, I meant there would be static tension across the crank due to bearing misalignment, rather than dynamic flex at RPM. This would sap power and increase wear on the mains.

Regarding flex at rpm, there is a very old 2-bearing in-line 4 engine that flexes like nobodies business. When people tried to reduce flex by adding an additional web and 3rd bearing in the centre, the cranks broke! So yes, definitely a not straightforward or obvious subject.
Tony

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Lambretta Sash
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Sun Jul 05, 2020 8:33 am

It is possible to get this done with the right equipment, I've been getting this done on all high hp motors I've built for last 8 yrs as matter of course and also engines with problems . One engine had an upgraded crank fitted from a std innocenti which then kept seizing, looking at the wear and heat marks you could sense the crank was not running true to bore, off it went to a good friend who found it was out by nearly 2mm, this is not the 1st casing I've seen this on either. Another was on a Spanish casing with the gasket face being out two ways
The good thing about getting this done is that you have very little wear and engines last longer

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