Bottom End Rebuild?

Hobot · Jan 12, 2018

If ya luck out to have perfect board flat clearance of stock 'Babbitt' metal shells on pristine CNC machined journals only a fool would take a razor to em - if - not, catch up on tediouis hobot babbit habit here...........>

I do not have an answer to the whereabouts of the 'standard', however, our standard is 75% of the load bearing area. As there is no way of measuring this area, its really just a matter of scraping the bearing until there is a sensible contact area.
I have not found many engineers who appreciate the purpose of scraping bearings. It does make a difference. The shaft runs with less friction and better lubrication because of the containment of lube oil in the hollows between the peaks. There is no machining techinique that I know of which can duplicate the contact pattern produced by bluing and scraping.
Regards, Joe Mc Cormack
more here
Broken link removed

peter12 · Jan 12, 2018

IMHO if you strip, clean, replace bearings, possibly replace camshaft you should have absolute peace of mind, something you won't get if you don't. think about the breather mods at the same time. Good chance oil seals will have hardened if left not used as well.

Hortons Norton · Jun 9, 2018

Anyone have a ballpark figure on how much dynamically balancing a Norton crank costs? But I would clean the trap just so I knew I was not going to have the sludge come loose and plug up anything. Not that expensive for bearings and shells.

dougmatson · Jun 10, 2018

I would check them at least. When I took my 75' Commando apart the all the nuts that hold the two cheeks to the balancer/flywheel were hand tight! Only 9K on the bike and had never been opened up. Check them!

mdt-son · Jun 10, 2018

Hortons Norton said:
Anyone have a ballpark figure on how much dynamically balancing a Norton crank costs?

In the UK, 120-150 GBP. No clue about US prices, but I guess at approx. $175. Select a renowned company over low price.

-Knut

Hortons Norton · Jun 10, 2018

Thanks Knut, Yes that's something I would not go cheap on.

SteveA · Jun 10, 2018

RoadScholar said:
Great opportunity to shot peen the rods and get the rotating/reciprocating parts dynamically balanced.

Lets say this bike will be ridden mainly on sunny Sundays. Shot peening old rods may save a fracture and could be worthwhile, perhaps good to do crack tests beforehand as well. After all they are over 40 years old.

On the other hand the bike has isolastics! It does not 'need' dynamic balancing to be a pleasant Sunday ride.

When someone is considering not even opening the cases as an option, it seems to me that continually raising the stakes isn't that helpful!

RoadScholar · Jun 10, 2018

SteveA said:
Lets say this bike will be ridden mainly on sunny Sundays. Shot peening old rods may save a fracture and could be worthwhile, perhaps good to do crack tests beforehand as well. After all they are over 40 years old.

On the other hand the bike has isolastics! It does not 'need' dynamic balancing to be a pleasant Sunday ride.

When someone is considering not even opening the cases as an option, it seems to me that continually raising the stakes isn't that helpful!

If you don't want to split the cases then dynamic balancing is not an option. I have 4 sets of previously run OE connecting rods 3 sets have pairs that are within 5 grams, one set has a pair that is 40 grams apart, never mind the 3 piece crank. The Isolastics insulate you from power unit vibrations, they do not insulate the engine from imbalance. The reason I have 4 sets of (used) OE connecting rods is that I do not trust old rods that have been through countless heat cycles under a wide range of start-up temperatures and loads. Magna fluxing and shot peening old rods, assuming they have round big ends and tight small ends, is a good half way measure. A good dynamic balancing brings your vibration plane close to a single vertical dimension as opposed to an unbalanced (unknown) engine where the vibration can be found in both vertical and horizontal directions, never mind the harmonics. Consider that a single gram of static imbalance can turn into 1.5Kg at a Norton's redline.

If you truly want this motorcycle to be your Sunday driver and don't anticipate exceeding 5K RPM often or at all, then you're probably going to be fine. For me it is a case of pay-me-now or pay-me-later (more).

SteveA · Jun 11, 2018

RoadScholar said:
If you don't want to split the cases then dynamic balancing is not an option. I have 4 sets of previously run OE connecting rods 3 sets have pairs that are within 5 grams, one set has a pair that is 40 grams apart, never mind the 3 piece crank. The Isolastics insulate you from power unit vibrations, they do not insulate the engine from imbalance. The reason I have 4 sets of (used) OE connecting rods is that I do not trust old rods that have been through countless heat cycles under a wide range of start-up temperatures and loads. Magna fluxing and shot peening old rods, assuming they have round big ends and tight small ends, is a good half way measure. A good dynamic balancing brings your vibration plane close to a single vertical dimension as opposed to an unbalanced (unknown) engine where the vibration can be found in both vertical and horizontal directions, never mind the harmonics. Consider that a single gram of static imbalance can turn into 1.5Kg at a Norton's redline.

If you truly want this motorcycle to be your Sunday driver and don't anticipate exceeding 5K RPM often or at all, then you're probably going to be fine. For me it is a case of pay-me-now or pay-me-later (more).

There is another point here with balancing, you need to select a balance factor based on the revs you are most often going to use. To do that you need to know how you are going to use it. If you have been riding it for some time that should be OK, if you have never ridden it before maybe not!

My crank was supplied and balanced by Steve Maney, to the factor we agreed suitable, it is, for a rigidly mounted motor in a race bike. It is also a static balance. This factor would not suit the OP's application, or yours.

Bottom End Rebuild?

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