Ignorant Rod Bearing question

68 Norton Commando (~9400 miles on clock)

Just got the crank(s) back from the machinist where he recommended a polishing rather than a .010 grind. He told me that I should use "STD" bearing shells and those are on order and will be here soon.

So my dad taught me long ago that you always doublecheck everyone's work regardless of how much you trust them so I got out my trusty mic and measured the crank journal's comparing it against the Commando workshop manual. Now I'm quoting from memory here so my apologies if I transposed number or got it wrong in my head but I think the manual shows 1.7505/1.7500. My mic is just a mid-level one so it doesn't go that fine, but what I was seeing was 1.750 on one, and the other I swear I was getting 1.752 or 1.753. (and I did measure multiple places both side/side and around to ensure I wasn't seeing out of round or some such) Is it common to have a crank journal that's BIGGER than spec? Any issues with fitting STD rod bearing shells?

(yes . . . .I know I'm probably overly paranoid but this is the first Norton motor I've ever rebuilt and they're pretty scarce in my part of the woods. )

Have a good one,
Dave

Did you check that your mic reads 0.000 when fully closed? If it is a 1 to 2 inch mic, you will need a 1 inch precision "Jo" block to check.

If your journal is truly +2 thou oversize, it would not be possible to spin the engine over (unless someone reamed out the old shell). Hard to believe it would leave the factory that way.

Slick

This comment is from Ken Canaga on another thread......


'....my Commando manual lists the standard big end size as 1.750", or 44.450 mm. The manual allows 1.7505" to 1.7500"/ or 44.450/44.463 mm.

Ken'

That confirms what you think the size should be is correct, and I think you are right to suspct your measuring/measuring tools first!

But.....

In any case, when you fit those new shells you will be checking for free play and movement as you torque up the new rod bolts and nuts you ordered won't you?

You did order new rod bolts and nuts right? They didn't do many miles, but they are over 40 years old, and what are you doing to check the integrity of the rods themselves?

Are you splitting the crank to clean the sludge trap?, and fitting new bolts, studs, nuts and tab washers there?

Steve

I would agree it is most likely to be your micrometer that needs recalibration. Originaly it would have come with a standard block or rod to reset it with but it you don't have one your local friendly machine shop may help you out.

More worrying to me would be the fact that the bearings are different sizes when checked with same micrometer. If the they are different then each rod an bearing most be kept on the appropriate side or it may be worth having crank ground to a standard size so you don't have to adjust the size of bearing shell.

Fit the rod shells and rod caps with rather less than full fastener toque then turn on crank, if much resistance felt, open to see witness marks on rod shells and use raxor blade @ 90' to carefully scrap off a few .0001"s and repeat til gravity just allows rods to turn on crank almost freely. If razor scraps through the soft surface layer then send back for more journal polishing or resizing sown for next szie thicker shells.

My mic is just a mid-level one so it doesn't go that fine, but what I was seeing was 1.750 on one, and the other I swear I was getting 1.752 or 1.753

Click to expand...

Are you sure you are not misreading the Mic and its actually 1.7502 to 1.7503, if it is 2 to 3 thou over then an assembled con rod with std shells will not turn.

Tel us (pic is better) about your measuring tool. Digital? Vernier? Brand? "Mid-level"?

First order is to measure the journals several times, repeatability is a big part of verifying accuracy.

OwhArrgh .

http://plastigauge.co.uk/



Ovality etc & wong filet wadius are worth consideration .
Worf wollieing about is ROD BOLTS . real live very genuine ones .
Cleaned in acetone , deburred etc . New Genuine nuts would be
worth hunting . :lol:
Loctite is mandatory .

Clean Threads , 2 to 3 drops ea. nut . f you dont waste any .

You DID clean the SLUDGE TRAP . Didnt you . ! :twisted: :mrgreen:

I do have standards so it's a good point to double check those and measure again. (I'll do that this weekend)

Yes - new rod bolts and nuts
Yes - sludge trap spic and span
Yes - new bolts, nuts, studs and keepers for crank.



Thanks for bearing with my paranoia (pun intended)