73 850 Commando head repair...>

I picked up this 850 two summers ago and had it overhauled winter 2013-2014 by a Brit bike/car guy in RI. The refurb included having the head sent out and
barrels painted. The head had new kibblewhite valves & springs installed, guy said guides were good and were not replaced. New rings went on the pistons
and the mechanic said there was virtually no wear on the cam. Bike had a little over 8k miles on it when I got it. I moved from RI to NC June of last year, so
I did not get much riding in on it. When I did, I found that I got smoking from the timing side. The smoking did not take place under acceleration, it became
noticeable when I'd stop for a light or to chat and it idled for a minute or two. Then it would smoke ( bluish ) and foul the plug. After thinking about this and
speaking with a few people, I came to the conclusion that it was more likely a head issue than a rings issue. So this winter I set up to pull the head. I had the
WW spanners but lacked the 1/4W socket. Finally got that in and set to it this past weekend. I've got it pulled and in the back of my car. I've got a good
recommendation for a head work guy over in Hendersonville, NC.

After pulling the head I examined the pistons. First question is just a curiosity, what do the markings mean? They are "ID 2", a "B" right in the center and "STD".
Next question - the fingernail crescent dings from the intake valves. I don't believe they were there when the head was pulled during refurb, but I can't swear
to it. The ding on the timing side cylinder is bigger than the one on the primary side. So, how did this come about? And could it be the cause of the timing
side smoking/plug fouling?

Additional info - bike came back to me with points setup. I've put in a Tri-Spark as I've also done with my '69 Bonneville. Stock coils, though I've purchased the
CNW coil replacement module and will be installing that when I set things back up. I'd like to have this beast running nicely for the Norton National which will
be taking place in Asheville, just an hour away from my house! Backstory - I learned on & rode Yamaha RD-350 & Suzuki GT-550 triple from 1970-1982. Got
married and the bikes went away. 30 years later and on my second marriage, wifey picked up the Bonneville for me as a birthday present! I have some
fairly basic wrenching skills, just replaced the clutch & cable on the Triumph, but I'm no where near the skill level of most of the contributors on this forum.
So be kind! Here's a "before" photo and pics of the dings as well as my work area. Thanks in advance!

ommief said:

After pulling the head I examined the pistons. First question is just a curiosity, what do the markings mean? They are "ID 2", a "B" right in the center and "STD".

Click to expand...

ID 2 = Hepolite code marking.
B = Bore grade size B.
STD = Standard bore.

Surprising that LAB didn't explore the dings in the tops of the pistons.

Something isn't right - they shouldn't appear there like that.
A sure sign the valves have been touching the pistons - not good.
Either the head has been skimmed ?, or the valves are longer ?, or the gasket is thinner, ?, or or or. ?
Cam timing may have been out ?

Probably need to flycut small clearance in the piston tops to cure that.
Or bad things will happen.

Smoking 850's always worth checking that the little valve guide seals on top of the inlet guides are good,
I had one 'shrivel up and fall off' a freshly rebuilt 850, and it caused light smoking and plug fouling on that side.
Replaced it, all good.

Could be the dreaded RH4 ???

When my bike smoked like that the cause was a loose valve guide.

Greg

I had similar smoking issues and found that it was simply the intake valve guide seals. Both dried out, one cracked in half the other no longer tight around the valve. At a couple of bucks each they were a welcome cheap fix.

I'd be interested to hear more from the resident experts on the marks left by the valves kissing the piston in ommief's photos. When I took the head off to do the seals, I found the same fingernail sized mark on the timing side piston from the intake valve. Valves are still straight, and a squish test showed I had .085" clearance between piston and valve when fully open (cold and hand cranked of course). It has a stock cam and no creative cam timing.

With that much clearance, what could cause this? Valve float from over revving on a missed shift?

The dreaded RH4 head.

A loose guide is something to consider as is the dreadef RH4 cracked head. Sometimes you can see the crack in the port with bare eyes or magnifying glass as the crack will run longitudinal along the port from the base of the guide.

As for the divets in the piston:
Skimmed head.
Combination of too thin a head gasket and no base gasket.
Valve timing was off during trial assembly
Upgraded higher lift cam.
Weak springs (not likely as you stated new Kibblewhite springs).
Improperly installed valve springs (incorrect spring installed height) allowing valve float.

Those nicks in the top of your pistons probably came about when the head was being installed and the push rods weren't aligned properly. You need to make sure that the push rods are in alignment before torquing the head. My motorcycle has those nicks in the piston as well. As a matter of fact right after purchasing my bike I removed the head and discovered that one of the push rods was even bent. Just because you discovered those nicks on the top of the piston doesn't mean that the last guy that assembled it was responsible. Your head could have been removed a number of times beforehand and could have been nicked on any of those occasions. On my motorcycle I have service receipts of the head gasket being replaced on 3 different occasions and there were only 7200 miles on the bike when I bought it.