T120 pre-unit drive side main roller bearing clearance - Triumph Forum: Triumph Rat Motorcycle Forums
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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 11-29-2012, 01:24 PM   #1 (permalink)
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T120 pre-unit drive side main roller bearing clearance

Hi folks,

Having requested a drive side main roller bearing for my pre-unit T120, I have received a RHP MRJA1.1/8JC3.

This has a C3 clearance.

Does anyone know if this is appropriate for this application, or should I be using a slightly tighter CN bearing?

Many thanks Dave
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Old 11-29-2012, 01:25 PM   #2 (permalink)
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I'd use the C3.
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Old 11-29-2012, 07:35 PM   #3 (permalink)
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CN is standard for the drive-side roller.
If the interference fit in the crankcase is very tight,sometimes (rarely) you might need a C3 so the bearing still turns freely.
Sometimes a C2 (tighter than CN) will be OK ,50/50 chance.
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Old 11-30-2012, 10:47 AM   #4 (permalink)
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Thanks Tritonthrasher and Mr Pete,

Tritonthrasher - you'd use C3 - is that from experience, or tech spec from a pre-unit manual, or something else?

Mr Pete - I've realised that you are a wealth of info and you supply wonderful detail. If you would give me your opinion following this next opinion/info I have received, it would be much appreciated.

I called the guys at TMS Nottingham (who are generally pretty knowledgable and straightforward). Many of their Triumph parts are made by Harris Performance, who have all the original technical drawings from Meriden. It is run by John Birch who was a technical man at Meriden 1960-80ish. According to TMS, he says that any T120 crank pre 1968 has main journals 0.0005" larger than post 68 cranks, and should run a C3 driveside roller, later cranks should run a C2 roller (I wonder if he meant a CN really, but I didn't think about it till after hanging up).

My crank is an early one, actually a 3-piece, amazingly still with nice STD bigend journals after all these years (I did the feed oil seal conversion right at the start in 1980 and have run a return filter for many years), so according to this source a C3 is right.

However, this variety of respected opinions makes me uncertain, to say the least!
I've never heard of this issue before.
If I use the C3 when a CN may have been more "perfect", do you consider that it will result in any adverse consequences eg longevity, vibes etc?

Many thanks Dave
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Old 11-30-2012, 12:47 PM   #5 (permalink)
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I think a C3 roller is what I put into my bike, many years ago, but I may be remembering wrongly.

If a C3 was what I had, to do the job now, I'd probably use it, or at least try it and see how the crank spun and/or rattled around.

Pete is more knowledgable and experienced than me, but he could be something of a perfectionist. Is this true Pete heheh?
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Old 11-30-2012, 01:32 PM   #6 (permalink)
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Hi Dave. Just exactly what bike are you talking about? Year and model?? Could you post a engine number? I believe there were some changes in the pre unit cranks???

Might help to look at JR Nelson books he lists changes by engine numbers and/ or the late Hughy Hancox services manuals especially if it is still dead standard.

Just my thoughts.
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Old 11-30-2012, 05:15 PM   #7 (permalink)
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Quote:
Originally Posted by koan58 View Post
According to TMS, he says that any T120 crank pre 1968 has main journals 0.0005" larger than post 68 cranks, and should run a C3 driveside roller, later cranks should run a C2 roller (I wonder if he meant a CN really, but I didn't think about it till after hanging up).

If I use the C3 when a CN may have been more "perfect", do you consider that it will result in any adverse consequences eg longevity, vibes etc?

I don't know of any change to mainshaft diameter.It's always been listed as 1.1247"-1.1250" on the drive-side,even for a T140 (well after '68).Trident was slightly smaller,1.1245"-1.1248".
CN roller was always fitted,until '76.
'76-onward used C2 (BUT SAME PART NUMBER !!).I think the difference was in the crankcase interference,not the shaft.The bearing ID is around 1.1247",so there was very little,if any,interference on the shaft.

Before fitting to crankcase or shaft,the bearing has a little extra internal clearance to allow for interference fits.It will tighten up with interference.
C2 has 0.0004"-0.008" clearance
CN has 0.0010"-0.0016"
C3 has 0.0018"-0.0022"

The biggest problem would be a bearing that doesn't have enough clearance when fitted,and won't turn freely.That's not good for bearing life,and likely to fail.
If it's too loose,you'll get some rattle/rumble when you back off at higher rpm.

If the bearing turns freely and has some small minute clearance,all is good.There will be more clearance when the crankcase is hot.

If it's tight or extra loose,you can use a bearing with more or less internal clearance.If the crankcase was an unusually tight fit on the bearing (possible,but unlikely),you may need a C3 roller.That would give you 0.0002"-0.0012" more clearance than a CN roller.
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Old 12-01-2012, 12:24 PM   #8 (permalink)
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Hey Dave, did you come right? You didn't say.
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Old 12-01-2012, 02:51 PM   #9 (permalink)
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Thanks all you helpful people. I have managed to get this question thru to John Birch of LFHarris, hope to hear back next week.
Bushy - it's a bit of a bitsa (in a triton), alternator 6T cases, 3-piece crank(59ish maybe?), 8-stud top end, been my main squeeze for 32 years.
Run ordinary roller driveside (CN I assume cos no markings other than number), it is the original one from 1980 I'm now replacing. I hear your thoughts! but I'd like to understand why some say one thing and some say another before I either fit the C3 or return it for exchange.
I value all of your opinions, sometimes history/experience improves upon original spec/design.
Thanks Dave
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Old 12-02-2012, 01:41 AM   #10 (permalink)
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Sweet, let us know of the out come, cheers.
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