Head questions

Last night I removed my cylinders and head to bore to .020. The head I removed is a replacement for the original that had the exhaust thread inserts pull out. I had gotten this head of Ebay, sent it to a friend that pronounced it good to go, so reused my rockers, etc, and just lapped new valves in it.

When I replaced the head, I had also pulled the cylinders to inspect, and found the standard pistons had to much clearance between the rings and groove, about .010 or more. The bores were close to the wear limit, but I went back with standard pistons just to get a few more miles before a rebore. Last summer, it would smoke on start up, and following riders would tell me it would smoke some on deceleration.as part of my prep for the INOA national, I thought I'd rebore and fix a few other small items.

However, when I pulled the head, the colour in the combustion chambers looks ok, but looking down the exhaust ports, things don't look so good. The left side has a brown dry lining in it, but the right side looks like it has been ceramic coated, black and glossy.



Left port;


Right port;


Terrible pics, I know.

The head of both valves look ok, dusty brown in appearance, but the port on the right is glossy. Oil leaking around the guide perhaps?

Looks oily to me. Oil rings or inlet guides on the RH side shot?
Constant smoke = rings. On backoff then accelaration = guides.
Ta.

Both those cylinders look like they have been burning oil, bigtime.
Worn rings/bore can do that.

You don't say what year this is, but some (later) years have little seals on the tops of the inlet guides,
to limit the oil going down the inlet guides. If the seals are displaced, gone, or worn, that could
partly account for some of the oil burning too, particularly on decceleration.
If its earlier year, worn guides could be some of the culprit.

Some years of heads are also known for cracking around the guides too,
which can let oil be sucked into the combustion chamber.
Check carefully, cracks can be hard to see, cold.

Have fun. !

norton73 said:

Click to expand...

You appear to have a mix match of valves there and they appear to be well too far set into the seats (warn out), at least on the exhaust side.
I think it is reasonable to eliminate doubt and rebuild the head (new guides, seats, valves). Bore the cylinder to the next size with pistons and rings of course.

You really have to many "if's" "and's" and "but's" going on there not to get that top end sorted properly.

1973 850. RH4 head

Valves are new black diamond, all 4 were replaced at the same time. The mismatch is probably a poor picture, as is the dark appearance of the combustion chamber. There is definitely some oil burning from getting past the rings, my bigger concern is if the guide on the right is loose in the head and oil creeping down. Looks like i would then need oversize guides.