Can worn isolastics increase engine wear or even damage?

I just bought a 1974 Commando 850 from the original owner. It has 32,000 miles on the speedo and the bike is vibrating at speeds above 50mph. The owner decided his riding days were behind him and sold the bike to me. I found the bottom rear engine plate bolt PN 06-3211 missing/sheared off. The engine was also leaving quite a bit of black oil on the ground after a ride, even though it had recently been changed, as in less than 100 miles ago. My uneducated guess is that the case sealing has been compromised where the bolt is missing and the blackness of the oil is the metal of the case and mounting plate wear being carried away by the oil? I'm sure the isolastics are toast, (he never replaced them) so that is one thing to do on the road back to a rideable bike.
The question before me and that I present to everyone is should I just replace what is broken and ride the bike or should I go all the way with a complete engine rebuild? The engine is not an oil burner and reads 150 lbs in each cylinder when I checked the compression. It runs strong and sounds healthy. I am yet to do a leak down test but probably will later today. This bike is in for extensive bench time anyway, it will need extensive cleaning, and a gear box overhaul is relatively cheap. Is this the time to rebuild before replacing the isolastics? Can worn isolastics hasten engine wear? Might there be unseen case damage or bearing issues from vibration? I honestly don't think the PO rode the bike very many miles with the excessive vibration but I don't know for sure. Thoughts?

Jonez69 said:

The question before me and that I present to everyone is should I just replace what is broken and ride the bike or should I go all the way with a complete engine rebuild?

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I'd suggest you replace what's broken first and see what effect it has. Note the engine mounting bolts should be high tensile. And change the Isos.

Leave the engine alone, sort the isolastics out to confirm the vibration is worn/unadjusted iso's and only then cure the oil leak.

Heres what you could do
if not already read the workshop manual
replace missing parts/hardware
clean the suspected oil leak areas, to see where the oil is coming out
check the iso clearances,check headsteady rubbers

Black engine oil likely caused inside engine. Could be your fuel mixture strength is too rich.
Dennis

Also is the vibration at 50 mph in all gears, in which case its not the engine or even the iso's. Or is it 50 mph in 4th and at lower speeds in each subsequent gear, when the iso's and engine are most likely as it related to engine revs and not road speed.

Jonez69 said:

I just bought a 1974 Commando 850 from the original owner. It has 32,000 miles on the speedo and the bike is vibrating at speeds above 50mph. The owner decided his riding days were behind him and sold the bike to me. I found the bottom rear engine plate bolt PN 06-3211 missing/sheared off. The engine was also leaving quite a bit of black oil on the ground after a ride, even though it had recently been changed, as in less than 100 miles ago. My uneducated guess is that the case sealing has been compromised where the bolt is missing and the blackness of the oil is the metal of the case and mounting plate wear being carried away by the oil? I'm sure the isolastics are toast, (he never replaced them) so that is one thing to do on the road back to a rideable bike.
The question before me and that I present to everyone is should I just replace what is broken and ride the bike or should I go all the way with a complete engine rebuild? The engine is not an oil burner and reads 150 lbs in each cylinder when I checked the compression. It runs strong and sounds healthy. I am yet to do a leak down test but probably will later today. This bike is in for extensive bench time anyway, it will need extensive cleaning, and a gear box overhaul is relatively cheap. Is this the time to rebuild before replacing the isolastics? Can worn isolastics hasten engine wear? Might there be unseen case damage or bearing issues from vibration? I honestly don't think the PO rode the bike very many miles with the excessive vibration but I don't know for sure. Thoughts?
View attachment 121737View attachment 121738

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Ummmm.... you seem to have one too many Nortons, and you are short a BSA...

hee hee

Nice grouping

Do all the things outlined above and (if it hasn't already been done) change the gearbox layshaft bearing/s

Black Oil....
I suspect the engine has wet sumped into the primary and is blowing out the countershaft.
Engine oil seeps into the primary, mixes with the clutch, turns black, excees oil weeps/blows out throught the countershaft.
Easy to tell, black oil drippping down the back side of the primary at the clutch.

I would..
1. Replace any missing/broken fasteners.
2. Tighten/torque inspect all fasteners including the Head steady. (Involves removing tank)
3. Change/service all oils. If excess oil comes out of primary, there it is. Spec is only 7 oz.
4. Service the clutch while it's a apart. This is a normal Norton thing.
5. Myself, I wouldn't mess with the Iso's....yet.... Check Iso clearances if you wish.
6. I would reassemble, test ride and go from there.

Nice find BTW, there are years of enjoyment in front of you............Maybe.............
This Website just may be the finest, Brand Specific website on the Net.
The knowledge and skill set here is beyond approach..

Welcome and Good ridings....

MichaelB said:

This Website just may be the finest, Brand Specific website on the Net.
The knowledge and skill set here is beyond approach..

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Well said.

Definitely true with regards to old Nortons. I was a complete Norton newb when I joined and the help has been immeasurable.

Less so if you’re into the modern ones… it’s handbags at dawn over on that page

Leave the motor alone, you say it goes good have good compression then leave it well be, fix the bolt and check out the Isos, adjust or upgrade to the newer ones, broken cradle/motor plates bolts or lose bolts will cause vibrations, 32k miles it's just run in lol, fix what need to be fix and ride it and enjoy it.

Fast Eddie said:

Well said.

Definitely true with regards to old Nortons. I was a complete Norton newb when I joined and the help has been immeasurable.

Less so if you’re into the modern ones… it’s handbags at dawn over on that page

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And it’s rematch number 4 if memory serves…

Thank you, everyone for the replies!