Datsun Roadster Forum-311s.org

What do you mean by undercut? And what is the remedy?

I haven't dug through the full posting, but I'm thinking more blown headgasket than a poorly seating valve. Maybe the gasket blew first followed by crud building up on the seat? Compression was good on the other three, so no concerns about rings ETC? Ya, since the head is off and you're planning to lap the valve anyway, may as well check the guides and valve stems for excessive wear. Again, didn't look through the post to see your level of knowledge or what access you have to appropriate tools etc, but obviously if you can disassemble it and lap the valve then you can do much of the head rebuild yourself.

You could try cleaning out the block and replacing/ inspecting the freeze plugs.
One method is to use a wire hanger and stick it thru a freeze plug hole to dredge up as much gunk and rust as you can and flush it out. Easier to do with engine out but has been done in place.
Undercut I believe means the holes are bigger than the gasket so might not seal correctly
Linda

Like Linda said undercut is when the coolant passage hole gets rusted bigger so it doesn't support the head gasket. The passage hole will often stay the same at the surface where it contacts the gasket but then when you look more carefully, you see that it is rusted bigger down underneath. A dental pic is good for exploring. Unfortunately the only cure is replacement.

I've confirmed that the lack of compression in #4 is due to a bent valve. I spoke with Dean to order a new valve, but he raised the question of "How did it get bent" and to figure that out before putting back together. He gave me some ideas on what to check. So far I can report that there is no damage to the cylinder, or the head. I've cleaned both the valves, pistons, and cylinders, and not seeing any thing that makes me believe that the valve made contact, doesn't appear to have sucked a nut or anything that would get into the cylinder and bounce around. The head measures out to 3.248, 3.238, 3.245, for the most part, taking measurements here and there on the head, so it looks like it's probably never been cut. The rocker arms move freely, and the valve slide in the guide smooth and free.

Any ideas on what else to look at?

Thanks again,

Ya, good question to ask... what bent the valve? Installed as a bent valve and not detected? With no apparent contact to the piston and no indication of foreign matter... hmm. Loose valve seat? I can't think of much else.

Got the head back together, never really saw anything that indicated contact with the valve. I'll be putting it back together soon and thought I'd ask if there is anything to watch out for. I saw another post where after the head went back on they were getting water in the block. Seems kinda straight forward, Head gasket goes on dry, torque in sequence, and adjust valves, right? None of the head bolts go through a water jacket, correct. I did notice that not all of my valve stem heights are the same, but should adjust out with the valve adjustment. Do you see anything I missed?

Are you using ARP studs or head bolts?
Linda

Re using the original head bolts.

You probably know that you can get a better torque with the ARP plus you don’t have to worry about snapping a head bolt, in case you were concerned about what you wrote above.
And this link concerns the one long bolt for the lifting hook:
viewtopic.php?p=70102#p70102

Linda

With the history of head gasket problems that these roadsters have no way I would go back with the original bolts. A mechanic who was working on my Ford F-350 showed me the difference between OEM and Arps. It was incredible.

It might be a loose valve seat. Did the new valve lap in properly?

Re ARP for those reading who are interested:
http://arp-bolts.com/p/FAQ.php
viewtopic.php?p=264702#p264702
ARP kit 154-4201

Linda

Cyclewrks wrote: ↑Fri Feb 21, 2020 12:33 pm It might be a loose valve seat. Did the new valve lap in properly?

I believe so, I got a nice ring on the seat and valve, throughout the head. I didn't notice any looseness in the seat or any odd sounds when lapping in the valve. Anything else I should be looking into before installing the head.

Are the valve seats brass or hardened seats? Some concern about brass seats and modern fuel.
Linda