Ouch, hurt the Datsun

[JT68]

Until all the data is in, I wouldn't get too bummed Greg.

What pistons did you use? You mentioned this was a recent rebuild? Are you sure the rings were fully seated? It can take quite a while.

Unless your ring lands are bad and you re-used tired used pistons, I wouldn't suspect the rings so much.

Leakdown testers are fine for detecting major issues, but not very good for discerning more subtle problems. Since you had air coming from the exhaust ports, that is pretty definitive, but rings/pistons/Hgasket, I'd say the jury is still out.,

Was there any oil smoke coming out the tailpipe?

Since the engine was running well before the near 7k excursion, I'd suggest just removing and repairing the head, put it back on and continue tinkering this year.

If the rings continue to seat, your compression numbers will improve.

If the compression numbers stay bad over 2021, its probably the pistons/bores. At least you would already have the head done and will know the whole story.

Either way, if you are going to stay with a z-196 cam, you should probably look at more serious springs. Most used/stock U20 springs won't be able to control the valve train at 7k with that cam(its the ramps that are the issue, not the rpm). Whether it happened this time or not, a valve float was probably in your future.

[Gregs672000]

Thank you for the input JT, it's always appreciated! Pistons are Arias, machined to "reduce" (ha!) compression, used (were new when I bought them) but perfect. New block was bored to them, then I worked with Total Seal to get the correct rings and carefully file gapped them. As you probably know, TS rings use a secondary inner ring that covers the ring gap (and thus the "total seal" claim, and I gotta say, #4 is solid, the gauge barely moves). I think the engine has seen about 3500 miles. That included 5 dyno pulls, the first where it was detonating as soon as I got on it before we pulled out some timing. The one after that was fine (as far as we could hear). We bumped up a couple degrees on the third and gained a few numbers but not enough to warrant the change vs safety. The table in it now is probably slightly different, and I need to take another look at the data logs to see what kind of load I'm seeing vs rpm and see if there are any concerning spikes or more timing than I want. There are changes that can be made in regards to what tables the controller references when determining load, and I recently switched that from a MAP based table to ITB mode which may be a little different (?).
It's a good thing I like doing this stuff, even if I screw it up sometimes!

[JT68]

Right, so back to what I said, 3500 is not a lot of miles, unless the Arias had lots of previous miles on them, I'd say there is hope for the lower end. (depends on many many factors).

I'd still say fix the head (correctly) and put it back together to see what you get.

[Gregs672000]

Surgery started yesterday and the patient is now resting quietly...
I pulled the head and confirmed two bent but not burned exhaust valves in 2 and 3, but what is interesting is that neither one shows initial signs of contact (?). A couple others have marked the block slightly (not like BEFORE, very very light) and they show no problems. You can't see any bend visually while installed, but a spray of carb cleaner into #3 exhaust and it will leak slightly while the others I could check with the cam installed are fully sealed. It's amazing, as you wouldn't think this small of a leak would show 50% leak down, but it DOES! Still not sure when the bending happened. Is it possible they were bent before when the known impact with the block was much higher, then passed the seal test I did before but rotated or something to cause more leak? I see no signs of piston damage or detonation.

So, plan is to disassemble the head, straighten the bent valves, check the springs, seats etc and reassemble. Still pulling the drive train to fix other things, but she's gonna be OK.

Thanks for the input, it helped me move forward.

[greydog]

How does one straighten a valve? Once straightened, will it stay straight?
Da n

[redroadster]

New ....cannot straighten 4140 steel valves
Maybe the seat moved too
3500 mi = 60 hours x 3000 rpm? = 180,000 revs ÷ 2 = 90,000 open &shut valve

[JT68]

New ....cannot straighten 4140 steel valves

Right, I hope Greg means "straighten out" as in replace. If its a minor bend, some shops will just try to grind a new face, that's a bandaid-especially on an exhaust valve- since you wind up with inadequate margin on one area, which then overheats and burns, so a year later the head comes back off again. New is the answer.

Its not likely at all that the valve was "always that way" otherwise, the engine always would have always run poorly since the last time the head was off. The valve contacted SOMETHING other than fuel/air otherwise it would not be bent. (and if it was, the head shop is really, really not adequate)

Also, if it was a insufficient piston or block clearance issue, the valve would have bent almost immediately 3500 miles ago. They floated. You need higher spring rate and maybe higher seat pressure with a z-196 and just about any other "race grind".

Also, FYI, there is minimal value and as you have found out, several drawbacks to oversizing the exhaust valve that much.

[redroadster]

Very true it's just short of a art form to get valves right
With the minute valve face of the imports especially
If your changing fuel set I would install a exhaust gas temp sensor on the header

[Gregs672000]

Again, thanks for all the input. The mystery continues... so after cleaning the ports etc as much as possible with various typical chemicals, brushing, compressed air, rotating the valves open and closed numerous times while cleaning and blasting them out trying to make sure all debris are out, I went through and tested each valve by rotating the cam until fully closed then spraying starting fluid into each one and watching for leaking past the valve. Results: #1 intake had an almost imperceptible leak, a tiny tiny seep that I'd never worry about; #2 no leaks, remember this had 25% leak down; #3 exhaust leaks worst of all, fairly steady seeping (this was the most obvious leak during leak down testing, 50%); #4 exhaust valve leaks, not as much as #3 but it's steady too (and this was the only cylinder that had NO leak down during testing). All other valves sealed and never leaked.

I bent valves in my 2000 Celica GTS by missing a shift at redline (3rd to 2nd, instead of 4th). Granted, they are much smaller, but they were obvious. This, not so much... thoughts? Carbon? There are no strikes I can see on the pistons.

[JT68]

There are only so many ways to say it, you need a proper valve job and correct spring rate/pressures to improve your performance and reliability. If I were you, I’d switch to a B or C cam and smaller exhaust valves for reliability reasons.

[Mattk]

Greg so sorry to hear about this. I hope that its minor and not major. i have faith that you will be bad on the road soon enough!

[Gregs672000]

Cleaned things better today but my spring compressor is a POS so I can't yet clean/check the valves, so I'll need to get another. But, ugh, damage to the head in #2 with melted metal between the seats (the raggedy spot, not the stakes). I would assume there will be some welding in my future. Others looked good. The back side of the ex valve looks different than the others too, likely melted aluminum. Will be dropping compression... expect about 10.3 to 1. It's not worth the 5hp loss.

[redroadster]

The way to test valves....is to turn the head down or bottom up ,fill the cyl with washer fluid , then use compressed air and a blow nozzle to try blowing air back thru the valve , no bubbles is super , 1/16 to 1/8 " good
3/16 " as long as it's a slow stream ok even 3/16" to 1/4"
is ok , but 3/8 and up needs work ,
the exhaust valve head is cooled by strong contact to the much cooler seat , of course check for grit , machine shops cut and narrow seats with a cutter reamer not grind and they sometimes came back with mixed bad results they saying it will straighten out in 2k miles ...but it was a lasting problem

[Gregs672000]

Got my spring compressor yesterday and pulled the two suspect exhaust valves... both were perfect and unbent! That's very good news. I cleaned all the carbon off of them and the port, then hand lapped and both the seats and valves look perfect. With the amount of carbon build up I'm beginning to suspect that I had some carbon holding up the valve and keeping it from seating fully. After I cleaned them up the first time (without disassembly, as above), I sprayed Seafoam into all valves, and both still had fluid in them a week later, and thats before i really cleaned and lapped them. The only bummer at this point is the detonation damage noted above. The steal seat is undamaged. I'm wondering if this area could be ignored or carefully smoothed out or if it requires welding, which would likely mean two new seats, cutting etc. Opinions? I can post or send better pics to any interested.

[Gregs672000]

Better pics.