While the head is off.....

Joined
Oct 11, 2019
Messages
10
Country flag
Good morning everyone...

I am the new owner of 1974 850 Roadster with 8,500mi. Picked it up last fall and have it running very well but one of the exhaust threads is shot and the head is now getting ready to go to Jim Comstock for thread repairs.
Since the head if off what else should I be looking for “while I am in there”?
Carbon buildup is minimal but a good layer of oil coated the front of the engine behind the crossover...will the new Head gasket fix that? The two nuts underneath the front of the head were not much more than finger tight.
 

lazyeye6

VIP MEMBER
Joined
Feb 28, 2014
Messages
762
Country flag
Have Jim strip the head and inspect the valves and valve guides. Do both exhaust threads so you don't have to send it back to him in the future.
Perhaps a valve and seat re-grind is due. Get the head media blasted to clean it up.
Of course a new head gasket but if the oil leak is coming from the push rod tunnel (often in manufacture, the head bolt thead bore broke through
into the push rod tunnel. This can easily be sealed up) rather than from a poorly sealing head gasket.
Sorry! I've potentially just spent $500 of your money. ;)
 

rvich

VIP MEMBER
Joined
Jul 25, 2009
Messages
3,124
Country flag
Rotate the pistons down into the bores and shine light in there and see if you can still see cross-hatching. Examine the bores at that time for any scars. If it isn't scarred and there is still cross hatching, I would leave it alone until a later date.

As for the head, talk to Jim about inserts for the studs. Maybe ask him about studs with a little extra length as well. The stock ones do not allow full thread engagement in the nuts. It gives piece of mind when working toward maximum torque on those fasteners.
 
Joined
Oct 11, 2019
Messages
10
Country flag
Of course a new head gasket but if the oil leak is coming from the push rod tunnel (often in manufacture, the head bolt thead bore broke through
into the push rod tunnel. This can easily be sealed up) rather than from a poorly sealing head gasket.
Sorry! I've potentially just spent $500 of your money. ;)
If I understand this scenario correctly I am looking for a head bolt thread breaking through the push rod tunnel?... would this happen in the cylinder block or the head?
 

texasSlick

VIP MEMBER
Joined
Jan 2, 2013
Messages
2,778
Country flag
If I understand this scenario correctly I am looking for a head bolt thread breaking through the push rod tunnel?... would this happen in the cylinder block or the head?
In the cylinder barrels ..... do a search, there was a thread on how to fix a few years back.

Slick
 

maylar

VIP MEMBER
Joined
May 13, 2007
Messages
2,642
Country flag
8500 miles is about when mine started smoking. Relacing the iron valves guides with brass fixed that.
 
Joined
Oct 11, 2019
Messages
10
Country flag
In the cylinder barrels ..... do a search, there was a thread on how to fix a few years back.

Slick
Found it! Clear as day, both short studs are loose and in the tunnel. Locktite or Thread Sealant?.... Thanks for the direction!
 
Joined
Oct 11, 2019
Messages
10
Country flag
Rotate the pistons down into the bores and shine light in there and see if you can still see cross-hatching. Examine the bores at that time for any scars. If it isn't scarred and there is still cross hatching, I would leave it alone until a later date.
Well the bores are not pristine anymore, and scuffing is light...but it is not smoking...yet. I am going to reassemble after the head is fixed and plan for a future winter rebuild. If the cylinders come off, the bottom end is just a step away.....
 
Joined
Oct 11, 2019
Messages
10
Country flag
[QUOTE="lazyeye6, post: 478677, member: 5985"
Perhaps a valve and seat re-grind is due. Get the head media blasted to clean it up.
[/QUOTE]

Placed the head upside down added Naptha and no leakage after 30 minutes....I should be good.
 

lazyeye6

VIP MEMBER
Joined
Feb 28, 2014
Messages
762
Country flag
With the studss in place, I had judicially sealed the small hole with JB Weld. Then removed the studs, applied some gasket sealer
to the threads and reinstalled them. I have found a better thread sealer/lock. Permatex 57535 Seal&Lock Thread Compound.
 

lazyeye6

VIP MEMBER
Joined
Feb 28, 2014
Messages
762
Country flag
Don't underestimate how easily oil can squeeze through nothing when under pressure from the crankcase.
 
Joined
Oct 11, 2019
Messages
10
Country flag
Appreciate all the good information, any suggestions on how clean the head gasket surfaces needs to be? Old composite gasket has left a veriy hard layer behind, assuming it needs to come off, any suggestions on how to soften it up?
 

gortnipper

VIP MEMBER
Joined
Nov 11, 2013
Messages
2,888
Country flag
Get Jim to do inserts on both your exhaust threads and the stud inserts too.

Are the plug holes good?

Definitely skim the mating surface.

What else are you doing to the motor? Any performance mods? I had him flow port the head, put in bigger valves and lighten the rockers.

Stonkingly good.
 
Joined
Oct 11, 2019
Messages
10
Country flag
Well....the barrels are going to Jim and new JCC pistons going in for one size over. The more I reflected on the sooted up left combustion chamber, and the scuff marks and very light pitting on that side, the more clear the decision has become.

Plus I get to peek at the camshafts and maybe more surprises!

Well even given all these expenses, I am still comfortably above water on this one. Not every day that you can find a very unmolested Commando sitting in a garage for $2500....original Candy apple paint and all.

As for the performance mods, I have no urge to get more power out of this bike, it has joined a stable of other 6 bikes most of which have a tendency to kill me quicker than the Norton.

I do love riding this bike though, the instant torque and good handling makes it comparable in riding sensation to my Ducati Monster and I understand its dedicated following.
 
Joined
May 11, 2010
Messages
2,204
Country flag
If your the head goes to Jim and you go for valve job, exhaust threads and the studs it will not be cheap. But it is a case of now or
later and later may be at an inconvenient and more expensive time.
More power is what we all want but more life and reliability has a LOT to be said for it.
 
Joined
Jun 9, 2008
Messages
1,349
Country flag
If your the head goes to Jim and you go for valve job, exhaust threads and the studs it will not be cheap. But it is a case of now or
later and later may be at an inconvenient and more expensive time.
More power is what we all want but more life and reliability has a LOT to be said for it.
The 850 will take more power and be reliable, bog standard 850's are under what a standard 750 could push out. Pushing the 850 just above 750 output is cheap, easy and still reliable.
 
Joined
Oct 11, 2019
Messages
10
Country flag
Well I found two gaskets and a compression spacer under the cylinder barrels, so replacing those with some Sealant is a guaranteed compression bump
 
Top