Motor mounting

L.A.B. · Mar 21, 2022

mdt-son said:
Reverting to my initial question, do owners nip up their engine studs and bolts regularly when high-tensile bolts are fitted?

No.

Fast Eddie · Mar 21, 2022

mdt-son said:
True, but is this scenario really applicable to the cranckcase/cradle connections of the Commando? I am assuming cradle has a thin layer of paint only. The crankcase joint should have a very thin smear of non-setting sealing compound. I understand the possibility of the compound flowing initially, but this flow will stop as pressure evens out (the compound does have compressive properties). The bolts are preloaded, right? So there is equilibrium between tensile force of the bolt(s) and compressive force(s) of the cradle lugs plus lugs at the crancase including the sealing compound layer. I can't see a reason for preload failure other than either nuts coming undone, or bolt material deforming by plasticity.

It's a fact that many so-called stainless bolts are cheap products made of AISI 304, which has limited staining properties actually. Looking at the stress strain curve at elevated temperatures (the bolts will heat up as crancase temperature rises - 75-85 deg C is to be expected - the stiffness gradient quickly approaches nil (0) as preload is increased beyond 400 MPa. Now, if the same preload is applied as for high-tensile bolts, say 60% of a bolt which yields at 800 MPa, the outcome will be a large plastic deformation and a diminishing preload. Repeated load cycles will result in a lasting elongation of the bolt. What happens when the preload is nil (0) is open to speculation. I guess the entire powerpack will twist and this could ultimately result in bolt rupture and damage to the crankcase lugs.

Reverting to my initial question, do owners nip up their engine studs and bolts regularly when high-tensile bolts are fitted? In my opinion, it shouldn't be necessary. If you do, do you replace the nyloc nuts regularly?

- Knut

It’s most definitely applicable if the cradle has thick paint or powder coat !

mdt-son · Mar 21, 2022

Fast Eddie said:
It’s most definitely applicable if the cradle has thick paint or powder coat !

Thick coating is contrary to my assumption, but all right. Cradles offered by A-N are powder coated if I remember correctly. Thickness of any powdercoated article varies from 20 microns to 100 microns. Perusing image of the cradle offered by AN, powdercoating thickness appear to be at the lower end of the scale, say 20-30 microns. Next up is a preload calculation where 30 micron layers of paint is included.

- Knut

marshg246 · Mar 21, 2022

mdt-son said:
Thick coating is contrary to my assumption, but all right. Cradles offered by A-N are powder coated if I remember correctly. Thickness of any powdercoated article varies from 20 microns to 100 microns. Perusing image of the cradle offered by AN, powdercoating thickness appear to be at the lower end of the scale, say 20-30 microns. Next up is a preload calculation where 30 micron layers of paint is included.

- Knut

I don't believe in 100% of this, but I do believe in the cradle, yokes, and swingarm parts: https://www.oldbritts.com/powder_coating.html

I used to powder coat cradles but IMHO, you must not have powder coat on either side of the cradle where the engine mounts, and you won't properly get the swingarm on if you have powder coat in that area. Scraping it off is difficult and masking before coating is a PITA. So, now paint them. The single stage, no primer required paint I use appears to be thinner than the paint the Norton factory used and way thinner than powder coat.

I'm building a bike for a guy right now who had the cradle power coated - I could not use it. Not only was it not masked and double-coated but it was clear coated as well. The coating is VERY thick. His swingarm was not masked either - took me four hours to make that useful. I like powder coat very much but it must be properly applied and proper masking must happen first.

kerinorton · Mar 22, 2022

LarryS.

If your engine to cradle bolts have come loose, you had best check to see if there is a lot of wear between the bolts and cradle holes and between bolts and engine holes. If there is, you will always have problems. You need to reduce that clearance to nothing. I had that problem and I cured it by fitting shim stock between bolts and the ID of all the holes. I ended up having to wind the bolts through as I had reduced the clearance to very tight. My engine bolts do not come loose now even though I did not use loctite.

The interesting result of doin that was I could not get the primary chain back on without loosening it off,. Fixing the cradle tightly caused the gearbox to move away from the engine.

I was recommended to bore the holes and fit oversize bolts, but I did not have the expertise to do that. What I have done has worked for at least 6 years now.

jan nelder · Mar 22, 2022

kerinorton said:
LarryS.

If your engine to cradle bolts have come loose, you had best check to see if there is a lot of wear between the bolts and cradle holes and between bolts and engine holes. If there is, you will always have problems. You need to reduce that clearance to nothing. I had that problem and I cured it by fitting shim stock between bolts and the ID of all the holes. I ended up having to wind the bolts through as I had reduced the clearance to very tight. My engine bolts do not come loose now even though I did not use loctite.

The interesting result of doin that was I could not get the primary chain back on without loosening it off,. Fixing the cradle tightly caused the gearbox to move away from the engine.

I was recommended to bore the holes and fit oversize bolts, but I did not have the expertise to do that. What I have done has worked for at least 6 years now.

luckily, standard metric bolts are slightly larger!

Onder · Mar 22, 2022

Nylocks and steel stop nuts are good for two on and off cycles. Nylocks don't keep things tight they just keep them from
bailing out. New stop nuts do a little better. Blue loctite to me is like a nylock. The higher grades are more like what FE says 'glue'
but then you don't want to retorque you will break the bond. I use loctite wicking for a lot of small stuff that you wish to keep
attached to the bike rather than things that require a specific torque.

Fast Eddie · Mar 22, 2022

mdt-son said:
Thick coating is contrary to my assumption, but all right. Cradles offered by A-N are powder coated if I remember correctly. Thickness of any powdercoated article varies from 20 microns to 100 microns. Perusing image of the cradle offered by AN, powdercoating thickness appear to be at the lower end of the scale, say 20-30 microns. Next up is a preload calculation where 30 micron layers of paint is included.

- Knut

I don’t think the issue is with new cradles !

And neither was there an issue with factory paint (very thin and very tough).

The issue is that these 50+ year old machines get painted and powder coated by all manner of people. Using all manner of products.

Its been discussed at length on here that there is nothing at all wrong with the ‘correct’ powder coatings applied by people who know what they’re doing.

The problem is that many powder coaters do not know what they’re doing and treat motorcyle parts no different to garden furniture. You cannot calculate against this as the variability is unkown.

The bottom line is simple to find on this forum: there are many stories of lose cradle fasteners happening after rebuilds with powder coated cradles (mine was one of them, and my cradle was powder coated by a VERY well respected Norton guy), there are many stories on here of PC causing lose cradle fasteners - which in turn leads to cracked crank cases.

So YES… it is extremely important to keep an eye on the cradle fasteners and keep nipping them up after a rebuild.

Larry S · Apr 13, 2022

Just wanted to post an update. I finally installed the new cradle bolts/nuts/washers and reassembled the primary. Filled it with oil and took it for a short ride, maybe five miles or so. After getting back to the shop I checked the bike over thoroughly and found that all cradle fasteners were nice and tight. Everything looked great. I then went on an extended ride, about twenty-five miles and then back to the shop for inspection. All fasteners tight and no leaks anywhere. Lesson learned is to use the correct fastener for the job intended. I will be checking these cradle bolts after every ride.

Dan1950 · Apr 14, 2022

Larry S said:
I would like to know if I’ll mess anything up with riding with just the headers with no silencers. I have read that you can burn up your exhaust valves if your headers are too short or too large of a diameter. Something about cold air hitting the valves ruining them. I didn’t completely understand it but it seems valid. Thoughts?

Thanks for all the help.

I once warped an exhaust valve in my '67 BSA A65T when I pulled off the interstate after an extended WOT run and immediately shut down the engine. As long as you don't do something similar I don't think you will have a problem.

I would advise against any extended WOT anyway as your A/F ratio might be off due to back pressure differences.

Larry S · Apr 14, 2022

Dan1950 said:
I once warped an exhaust valve in my '67 BSA A65T when I pulled off the interstate after an extended WOT run and immediately shut down the engine. As long as you don't do something similar I don't think you will have a problem.

I would advise against any extended WOT anyway as your A/F ratio might be off due to back pressure differences.

No worries now as I’ve fit both silencers.

Motor mounting

L.A.B.

Moderator

Fast Eddie

mdt-son

marshg246

kerinorton

jan nelder

Onder

Fast Eddie

Larry S

Dan1950

1974 MK II Roadster

Larry S

Similar threads