Crankcases gap? Normal??

Question for you guys,

I was just about to start re-assembling the crank cases, found a gap that runs the whole length of the underside. I know the 2 screws + bottom bolts + welseal will pull them together a bit...But even so! :shock:

Thoughts?

Steve

I'll see if I can post a piccy shortly

The mating surfaces around the rest of the cases are a perfect fit, it's what is at the bottom is causing me a bit of consternation. Again seems to run from the bottom bolt hole on the front isolastic to the bottom bolt of where it bolts to the subframe.

I would make sure the recesses where the dowels fit in are completely clean. I had to get into mine with a small pick and clean the edge where the dowel sits, and then I took emery cloth to the dowel to make sure it didn't have corrosion or a burr. Once I had done that, mine slid together with click when the faces met.

Russ

That gap has to disappear completely BEFORE any clamping forces are applied.

A lot of interference (bumping) can occur during a rebuild and bringing these halve together was a scary part event.
Many dry fits were done before the crank was inserted along with pulling the dowels and replacing with fresh items. A long heavy mill file was laid across were possible and with a "delicate" touch the surface was confirmed, so to speak.

There is much pride associated with having a dry Norton that you have personally built. With due diligence and caution, I have a dry crank case.

Thick glass plate with 600 grit wet sandpaper to check for flatness of cases. A few strokes pushing and pulling the case over the sandpaper will reveal any high or low spots.

As I recall, my 850 cases had areas with a raised ridge that fit into the opposite case, so that they overlapped a bit. For this reason I didn't use sandpaper on glass like I do on lots of stuff. But it was another area where I found tiny bits of old sealant that came away when I ran the tip of the pick along it. The stuff wasn't easy to see until it came away. It was no longer soft and pliable to the touch but more like brittle plastic from age. You want to make sure the gap isn't caused by the crank before you start torquing it down!

I had a similar gap in my cases when I built my basket case into a road going bike back in '83. The gap in mine was near the top rear of the cases, and I know why it was there:- the cases had been broken in several places and welded back together.
While I have used the sandpaper on glass method for other parts, that was not an option here, because the problem was a gap that needed to be filled, rather than vice versa. Besides, the lip on the drive side case makes sanding it on a plate impractical.
To fill the gap I made a thin, tapering aluminium shim and glued it to the drive side case with araldite. Don't laugh, it worked and was oil tight.
As grandpaul stressed, the gap must be eliminated BEFORE trying to clamp the cases together. A gap is not normal.

Another thing that you must consider is if you have not seen the cases together before, and observed that there was no gap there, is that they are 2 different cases off 2 different bikes.

There is gaps and gaps...thats some gap :!: ensure the hole edges are slightly countersunk, perticular the threaded holes as they can be pulled up.

Hegel said:

The mating surfaces around the rest of the cases are a perfect fit, it's what is at the bottom is causing me a bit of consternation. Again seems to run from the bottom bolt hole on the front isolastic to the bottom bolt of where it bolts to the subframe.

Click to expand...

Bernhard said:

Another thing that you must consider is if you have not seen the cases together before, and observed that there was no gap there, is that they are 2 different cases off 2 different bikes.

Click to expand...

A matched pair of cases are normally stamped with the same number.

I would have thought None-matching cases would still be flat on the face's, just the bores and stud holes would be out?

i've seen this after welding repair of cases and have to beat the living snot out of the beads and surrounds to ever so gradually mold the gaps back to full seating or at most a minimal .003" gap at the top seam hat good sealant with good thread back up - sealed fine. Ink the dowels and remove any witness marks on them. Then cold brutal hammering with something like course sand paper between to impress a more factory texture than beat up with a ball ping hammer head.

Steve, that beating rough sandpaper onto the surface is an old trick to disguise re-stanping or weld joins :roll: looks like sand cast finnish when done right.

Yes ancient practice to get sealed cases again w/o advertising to close onlookers. If there is nothing else preventing cases from intimate mating what's left but brutal will power to ride again oil tight-ish. In my cases I mostly knew where to beat and beat and beat... d/t the 18" of melted weld area. In these cases I'm not sure where to beat but by eyeball and feeler blades to guide one's strikes, realizing that closing one area can open another like tuning a wooden framed piano, back and forth with only lack of money for new cases and desire to ride a fully fettered Commando again powering the next strike in the dark...

Hi guys,

Many thanks for the replies

Seems to have been a combination of several factors, the first being the dowels holes/ threads were still full of crap from the vaqua blasting I had done on the cases. Fortunately for me there is a good sized ultra-sonic tank where I work, was able to drop them in and let them fizz for a while. The down side to the dip in the tank is that the fluid used stained the cases a blotchy gun metal color. :roll: So then had to spend the next few hours trying to get them back to some sort of uniform color. It was weird, as I've used this tank to clean up other parts and they've come bright and shiny..Dunno.

I think the other factor was until I'd screwed in the long studs, got them seated in straight, they were pulling the cases very slightly. The upshot is they now seem to fit together nicely. I did a trial fit of all the case screws and bolts (including the front and rear isolastic bolts), there are no gaps at all. Whew!

I managed to get the cases heated up and put the mains in. Next job is to assemble the crank. I thought it prudent to unpack and check the barrels, only to find that post honing no-one had thought to drop a bit of oil down the bores to protect them, they've freakin rusted! It's not as if they'd been left in a damp place, like the garage. I'd kept them in the house, carefully packed away. So they've got to go back for another hone.

Oh...And while I'm having a moan...I bought a brand new front isolastic case....It doesn't fit. It properly doesn't fit (not just excessive powder coating), so that has to go back and get exchanged. I'll take a pic and post it.

Sheesh talk about some hard lessons to learn. :evil:

Welcome to british owership, reading last night about 1974-76 factory commadon production ..less said the better! But glad i still own two...i think.
http://www.lansdowne-engineering.com

What were you reading, share, we need to be mortified.

Hot dog for figuring out the case hang ups. The light bore rust may not be an issue if just lightly sanded smooth-ish and slight pitted texture may help hold oil to seal better. Just make sure the ring gaps are right, ugh. The case covers are essentially pure Al but case alloy and head alloy have elements that make them darker all the way through. Some case came painted to that's an option to consider.