Crankshaft main bearings

Hi Jim

I'm responding to try and assist but can I suggest that you pm one of the heavy twin experts like Phil Hannam. I'm more of a Commando chap - well for motorcycling anyway - but know a bit about bearings.

Please can you indicate the engine capacity and model. I'm guessing it's a 650 built at Norton Plumstead.

Also indicate the precise manufacturer's specification, e.g. any numbering on the NSK bearing supplied under part number SS08 D3-17824 by NOC spares.

I'm familiar with the FAG bearing SS08 D3-064118C3.  This is the bearing often marketed as superblend but it is actually a completely standard bearing.  But that's another story.

If you let me have the NSK details I'll take a look but it would be good if one of the experts on applicability for your engine chipped in.

Thanks, Andy

hello, now you sill can get the original bears R&M  Ransom and Marles MJ30 Ball race timing side or MJ330, Hoffmann or FAG 6306ball race,  then for the drive side  R&M MRJA30. or MRJA30-6B  super blend roller race  bronze cage  or FAG NJ306-E super blend roller  brass cages,  NSK bearing are made in East Germany and are very good  bearings    as are SKF Timken Bearings   just some to be going on with    yours  a\nna j  

I'm a bit surprised that no-one yet has mentioned the excellent article elsewhere on this NOC site re: main bearings.  It's a convenient (!) 32 pages and is very comprehensive.  Ignore the word "Superblend" as it's virtually meaningless - as this document implies.  I favour the barrel-shaped section rollers as they make more engineering sense in permitting a degree of crankshaft flex compared with the plain cylinder type - but hey, what do I know?  I believe there is a needle roller conversion available for the timing side, but I stand to be corrected.

Actually I'm a retired Civil Engineer and we are the oldest profession - apart from the Biblical one - so we did construction ("Civil"), structural, electrical and mechanical engineering before those professions were invented.  So I have forgotten more than most people will ever learn!  LOL!

Jim, thanks for providing details.

There are quite a few factors at play in selecting main bearings particularly as so much has moved on since the hiatus with Norton Commando engines in 1972.

The document Lionel mentioned is intended as a detailed reference to substantiate the summaries provided by the author, i.e. me, in two more digestible articles in Roadholder (No. 352 April 2017 and No. 366 June 2018).  There is so much opinion and much less fact around on all this that I wanted to try and take a scientific approach! Many people contributed to the effort including other NOC members.

The detailed NOC reference document is here https://www.nortonownersclub.org/sites/default/files/Main-Bearings-v1.1.compressed.pdf

Turning to the bearings you have, the NSK NF306 is of the type listed as an alternative fit to the Ransome & Marles MRJA30 (pre "superblend") in Norton Service Release 68 of October 1971 (see NOC reference doc Pg 24).

I am a little pressed for time at the moment - getting ready to move house - but a simple way to assess an important part of suitability is to look at the dynamic load rating for a bearing.

The dynamic loading for today's NSK NF306 is shown as 38,500N (page B88 of the NSK catalogue).  If you take a look at the graph on page 28 of the NOC reference document you will see where it fits in. By way of contrast FAG NJ306E bearing data for 2014 showed a dynamic loading of 61,000N.

This is fine as far as it goes but what is really needed here is practical experience. All my knowledge and data are based on the Commando.  Although there are many similarities there are also differences, e.g. additional webbing on later Commando crankcases.

If it was my bike I'd probably fit two of the NJ306E bearings and investigate shimming the end float. As Anna says I'm not sure the make of bearing is critical. Having said that it's possible the NSK NF306E will do the job. 

Much of the problem with the Commando's mains was the Combat engined version with a higher state of tune and the invitation to regularly rev it to 7000 rpm!

Hope this helps and that one of the experienced high mileage 650 owners chips in.  This will be more relevant than any theoretical analysis.  I'm also not keen on racer's experience on this subject as their annual mileages are usually low with engine rebuilds every season.  

All imho, Andy

Hello its no Dought that NSK bearings will do the job. The thing is that have steel cage and how long that will last is any ones gess but only time will tell.   Yours Anna J