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sam christie

United Kingdom
3108 Posts
Posted - 24/01/2021 :  17:52:45  Show Profile






Sam
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Simon Johnston

United Kingdom
6145 Posts
Posted - 24/01/2021 :  19:30:47  Show Profile
The change at engine 2501 AJ was the addition of a second bearing at the front of the crankshaft to “eliminate slight roughness at certain engine speeds” - see Service Information Sheet 27 here - https://www.triple-mregister.org/uploads/retro/service_inf_sheets.pdf

J3574 was fitted with engine 2501 AJ

Simon J
J3437
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Oz34

United Kingdom
2546 Posts
Posted - 24/01/2021 :  20:32:21  Show Profile
Thanks Simon. Clearly a fairly substantial mod which justified the change of number.

Dave
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Simon Johnston

United Kingdom
6145 Posts
Posted - 24/01/2021 :  20:48:42  Show Profile
Substantial, but not a proven benefit. I don’t think anyone adds the additional bearing these days.

Simon J
J3437
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Blue M

United Kingdom
1474 Posts
Posted - 24/01/2021 :  22:56:44  Show Profile
Someone said it led to broken cranks so they dropped the idea. But it seems they are now being reproduced so perhaps it isn't a problem with billet crankshafts.
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Simon Johnston

United Kingdom
6145 Posts
Posted - 24/01/2021 :  23:38:40  Show Profile
I think that the original cranks had a degree of 'whip' in them which would cause the very front of the crank to flex slightly. Fitting the extra bearing made the whole thing too rigid with the result that the crank broke rather than flex. I can’t see what benefit the extra bearing would bring to a steel crank installation.

Simon J
J3437
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Blue M

United Kingdom
1474 Posts
Posted - 25/01/2021 :  10:34:03  Show Profile
Simon, yes, since they don't flex so the extra bearing is unnecessary.
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JohnE

United Kingdom
374 Posts
Posted - 25/01/2021 :  11:58:34  Show Profile
If you have the extra bearing, you can use a cheap single row main bearing and take up the axial play with the front bearing shims.

JohnE
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