No shims behind Super Blend Bearings

Dances with Shrapnel said:

Al-otment said:

This could happen. But fitted properly, and especially on the timing side, is highly unlikely if not impossible. The smaller shims may well have been cheaper, but not by any significant amount.

Click to expand...

This does happen. We have reports of shims going AWOL, and I have seen three separate instances of shims offset and pinched tight into the filet; one of these instances was my own billet crank where the shim was confirmed concentric and tight on install but pinched upon dismantle.

A quick look at the pricing of Arbor Shims indicates a smaller shim is about 50% the price of a larger shim.

So yes, if properly installed on the timing side (tight and centered), I also would expect the shims to stay in place on the timing side. I cannot recall where the shims that I observed being pinched came from. Maybe Jim can chime in here to see if he recalls which side the AWOL shims came out of.

Click to expand...

My emphasis was on properly fitted shims, anything fitted badly is going to be a liability. Even with a 50% price difference on shim packs, the % difference in the bikes selling price and profit is going to be negligible, not including the fact that the Norton buyers would have got discount on bulk orders, so I don't think cost was a factor.

Yes, so my emphasis is that even fitted properly they come loose and there is a liability with that. Only mitigation might be placing shims on the timing side only with hopes that the tightening of the oil pump pinion will keep everything in place. I've not tested it and at this point see no reason for me to do so when good/better alternatives are available - albeit more costly.

My point being, with all the problems observed with shims on the main shaft and the dynamic nature of thin shims on a turning main shaft, why would one expose themselves to the risk. Is the answer "because that is the way we always did it" or "that is the way Norton did it". Not being disrepectful, just pointing out there were problems observed and even in one known good installation. This is just my opinion here but not everyone has an arbor press so dropping a predetermined thickness round shim into a warm engine case and then dropping in and then weighting down the bearing does not get much simpler. I never had a problem with this approach, never observed a problem in the shim performance and it is readily doable without any special equiment.

When I am building a performance engine I do a trial assembly with pistons that have had the top cut off so I can see the position of the rod on the wristpin. I will push the crank to the position that best centers the rods under the piston. That means I may need to install shims on either side.

Engines for high performance get shimmed at .020 end play. At high speed the crank flexes and gets wider. At .010 end play I have seen damage to the ends of the rollers from excessive thrust when run on the racetrack. Jim

OK so its a known no-no to shim too close. Is there a known risk-reason to just leave shims out and let crank self position w/o issue, hi speed use or not?

hobot said:

OK so its a known no-no to shim too close. Is there a known risk-reason to just leave shims out and let crank self position w/o issue, hi speed use or not?

Click to expand...

If the crank had way too much end float I would be concerned. I have seen them with .060 with no shims.
I would be concerned that the small end of the rod could be way off center under the piston which would cause uneven loading and excessive wear. Jim

Ok its known Einstein was a fraud scientist misapplying others equations and making up stuff to fit his ideas > so ask what factors/forces might over come the rods thrusts and drives attached self centering to require shims, > even if don't FEEL right sitting still hand shifting w/o shims. ?? Note I did not state the two types of relativity theory are wrong, just not really Einstein's own work and not as comprehensive as who he stole from. Only thing bad hair day Einstein did was prove particle-wave nature of sub atomic particles and photons, ie: photo-electric effect, aka solar cells and sails, that led other real researchers-theorists to quantum theory that he spent most of his life trying to prove was all wrong.


From my own meager observations> its not the centrifugal forces direxctly busting things out, so much as its the bowing of crank levering races against too tight side clearances so something got to give at some point that might not if room to flex provided for. Similar to isolastic gaps.

Just a thought of mine to add as I am waiting for my crank to return from grinding an balancing so all this talk of shims etc. is of great interest as soon I may be have to decide what my crank may or may not need.

The crank will flex at speed this is a fact as Norton themselves fitted bearing that could allow for this.
So I would imagine (tell me if am wrong here) that because of the position of big ends an bolt up nature of crank that it would bend in an arch type shape. That would make the big ends at a very very shallow angle an so would the rod not travel towards the center of the crank (flywheel) it being the high point??

Or is it the case that the crank bends in more than one plan depending on what part of stroke it is on loading on drive side an other such factors

toppy said:

Just a thought of mine to add as I am waiting for my crank to return from grinding an balancing so all this talk of shims etc. is of great interest as soon I may be have to decide what my crank may or may not need.

The crank will flex at speed this is a fact as Norton themselves fitted bearing that could allow for this.
So I would imagine (tell me if am wrong here) that because of the position of big ends an bolt up nature of crank that it would bend in an arch type shape. That would make the big ends at a very very shallow angle an so would the rod not travel towards the center of the crank (flywheel) it being the high point??

Or is it the case that the crank bends in more than one plan depending on what part of stroke it is on loading on drive side an other such factors

Click to expand...

At 0 degrees and 180 degrees the crank would bow and become narrower. At 90 degrees and 270 degrees the counterweights would be unopposed so the crank would flex the other direction and spread the outer flywheels to become wider. Jim

Thought it would not be a simple problem Norton did not seem to like making bikes with simple problems. They do seem to like shims though. I bought may Commando from I friend an one of he's reasons for not liking it was all the shims and opinions of how much to shims cranks, layshaft, ISO etc.

I've seen what happens, the crank goes in and out of a jump rope shape, so its center section tends to hang up at TDC on piston inertia placing great upward load on the case bores while the crank ends past bearings angle downward. This puts race tipping leverage outward force on top of bearing race/bores and inward leverage force on their bottom parts. As the materials are relatively elastic at these energy levels, there is a slight delay of piston stop/start inertia holding up crank center, so most bowing occurs just after TDC - which angles the ends past the bearings to aim most out of line at 5 to 4 o'clock region of cone shaped end arc pivoting in the bearing of course. Somewhere after 10,000 this can amount to at least 1/8" run out on DS. Implies rev limiter more important than shim space to me. This jump rope shape with flywheel at center also puts severe separation-leverage loads on the cheeks staying flat against flywheel faces by some bolt stretching rather more than rotational shear loads to resist.

So what happens if crank is left free to shift for itself?