How to determine if Main Bearings need replacing?

I'm restoring a '74 MkIIA with 28k on the mile-o-meter. My engine has the FAG superblend mains (Rollers marked: NU306E; races marked: NJ306E). Is there a reliable way to determine if these need replacing?

A new set of superblends will set me back a couple of centibucks, which I could spend on some needier section of the bike. Of course, $200 is cheap insurance against another total teardown IF my originals ever go (and this bike will henceforth live a coddled life). Overall the engine shows almost no wear after careful measurements of critical wear components. It apparently was not hot-rodded much if at all. What would YOU do?

Huffer
I will tell you what I did, but remember it is only my opinion. Others may have more experience than me with Commandos.
I restored a 74' 850 with 10000 miles on the clock.
I stripped off the head and barrels, and took the pistons off.
I protected the rods from contact damage with the casing and carefully rotated the crank whilst holding the rods. It felt smooth and good. I pumped oil through the crank and it came out clean and with good flow through both big ends. There was no noticable play in the big ends. When I allowed them to drop they moved smooth and slow on rotation.
I put a dti clock on the end of the crank and checked end float. It was about 0.008", so I was happy. I put a clock on the Crank jounals and tried to wag it up and down, using the rods to pull, and again There was little play, if any at all...maybe less than 0.0003" on either side.
I also checked the rods to see if they were 'nicked' or marked with a 'D' and checked valve lift.
So I concluded the bottom end was OK...
I bought new rings and gudgeon pins with clips and put the bottom end back together, after deglazing the bores.
However, I have to say the engine smokes on the overrun on the LH pot, but I think that is because my cylinder head builder damaged a new guide when he was installing them.

You have the same model as me....It will have come with superblends as standard from new, I believe, and also spent its life running with a spin-on cartridge oil filter....which is good.

Figure 28000 miles is quite a bit less than 1000 hours of operation. Not specific to a motorcycle but that's not much life for a roller bearing so you wouldn't normally expect it to be worn out. The cleanliness of the lubrication has more to do with it than anything, i.e. small particles going through the bearing can cause damage.

Superblends last a looong time if not abused. If the rollers aren't purple and there's no signs of pits (under a magnifier) they should be fine.

That being said, I replaced mine anyway because I never want to be inside the crank cases again.

A perfect used bearing race will be a mat silver with no pit marks. Inspect it very closely, particularly the inner race. I say that because I have just had to replace a split roller that looked good in the outer race, but the inner race was shot. Oh, look at the rollers as well. These main bearings do last a long time though.

Thanks for al the replies. My bearings look like they're new--shiny, no pitting, no play between the race and the roller. I'm going to keep them and spend the money on other bits. It's nice to have a '74 MkIIA with most of the prior design boo boos sorted out.

Hi Huffer,

I'm in the same situation as you - '74 MkIIA, 11K on the clock, and the only engine mods on mine were applied by the PO in the form of a screwdriver to attempt removing the head and barrels without first removing the relevant bolts & nuts :shock:
Internally the motor was very good apart from a soft cam - is yours OK?
Anyway, I've decided to keep the original superblends, but I am replacing the big-end shells because I wanted to inspect the journals, and the shells are guaranteed to have some scoring. That said, I could have refitted the original shells without any worries.

Andy,
I am indeed replacing the big end bearings, which are indeed very slightly scored. The crank journals look fine, are still within spec, and I'm not going to polish or otherwise fiddle with them.

Not sure what "soft" means in relation to the cam. What should I look for? Thanks.

Huffer said:

Andy,
I am indeed replacing the big end bearings, which are indeed very slightly scored. The crank journals look fine, are still within spec, and I'm not going to polish or otherwise fiddle with them.

Not sure what "soft" means in relation to the cam. What should I look for? Thanks.

Click to expand...

Some camshafts were basically duff and wore out very quickly get a micrometer and measure them and check against the relevant sizes from the workshop manual. There is a post on the board with a photo of a very badly worn camshaft somewhere ....

http://www.accessnorton.com/post38007.html?hilit=worn camshaft#p38007

Wow, those cams are scary looking. Fortunately, mine is apparently OK.