1FZ Lost intake valve clearance 6k miles after adjustment

[DesertLake]

Early last year, my 1FZ developed poor compression on two cylinders cold, but the problem seemed to get better after warming up.
I could hear it in the starter.
When I checked valve clearance, I had two intake valves at zero, and several others were out of range.
This is a motor I rebuilt 30k miles and 7 years prior, and I honestly don't remember checking the valve clearance or if the shop that assembled my head adjusted them.
So, I figured it was just due.
I ordered a few shims, mixed and matched the rest, re-checked clearances, re-checked compression, all was good.
I also posted up the shim part number table, linked here for reference
1FZ valve shim part numbers - https://forum.ih8mud.com/threads/1fz-valve-shim-part-numbers.1252957/

6k miles later, I lost cold compression on #1 cylinder again. Exact same behavior.

Re-checked the valve clearances, and the rear intake valve on #1 is down to zero clearance again.

I'm starting to unbolt everything from the head now, and I'll see what's failing soon enough, but I'm curious what has been seen already.

Thanks

 

[DesertLake]

102 views so far and nobody's had a valve rapidly lose clearance?

And man, I forgot what a pain pulling the head is!

 

[beatle_bayly]

Have you conducted a leakdown test? Compression may be OK after you've redone the clearances but running for a while at zero gap may have burnt a valve or damaged a valve seat.

Why was the engine rebuilt? Was it due to overheating?
When you rebuilt the engine did you fit new valves?

Sounds to me you either have valve seat recession or a stretching valve. Though to have this on the inlet side seems less likely than on the exhaust side.

 

[Kernal]

Were the valves replaced?

IIRC there was one person who rebuilt his 1FZFE but the machine shop installed aftermarket made-in-Chynna valves which were too soft and ended up becoming deformed (indented) where they mated to the valve seats so the clearances went out of spec after ?? a year or so IIRC. Apparently it was a US brand name who had farmed out manufacturing of the part to a factory in Chynna.

FWIW

 

[DesertLake]

Original valves. Rebuild was due to long term but small HG leaks leading to electrolysis pitting, so some metal had to be taken off the head and block. I did replace bearings, rings, etc even though the originals were still pretty good. Valve seals were leaking, so those got replaced. All else looked good at the time.

No leakdown test was done when I adjusted the valves last year, just compression.

I did try a borescope that can turn around and look back almost 180 degrees, to my untrained eye the valve didn't look bad but I didn't keep the pics.

The head is now almost ready to pull, so I should have a better view next weekend.

 

[robnicko]

Once you have that head removed should hopefully be evident what the issue is, perhaps it could be a couple of valve springs out of spec on #1 cyl / valve sticking on the guide?

 

[DesertLake]

I went ahead and bought a new set of valves and had a shop do the head work. The problem wasn't obvious to me, but the shop said those same seats needed some work. When I get time I'll try to get some measurements of the bad valves.