06/11/2026
Mystery 496 project update, not-so-Reader's Digest version of the situation;
When I finished final assembly a few months ago, I had set the valves at .016" cold lash. That is the hot setting, and I was anticipating it would of course loosen up a few thousandths once it lit up and got up to temp--I was planning on re-setting them for hot spec at that point as per the norm.
The engine had other ideas.
While Mike and I were in the process of pre-lubing the engine before initial start, Mike noticed something was "off" and asked me "did you set the lash on this?" (I was manning the starter button on the test stand while Mike was manning the drill on the oil pump drive).
I of course said yes, and he said that it appeared there was no lash present--at all--on any of the valves on his (passenger) side.
We paused pre-lubing the engine and I went back over all of the valves. Much to my EXTREME dismay and puzzlement, I noted that all 16 valves had closed up to 0 lash.
That was another first for both myself and Mike.
I have lashed valves more times that I can begin to count over the last 40+ years, and I know damn good and well I didn't bungle setting them this time, esp. not all 16 of them.
I went ahead and re-set all the valves to .016" again and we finished up pre-lubing and commenced with initial fire-up, which I posted the video of a few weeks ago.
After discussing this extensively with Mike, the only plausible conclusion we could come up with is that the heads must have lost their heat treat after being welded on who knows-how many times over the years and were moving around.
There was another situation prior to this where it appeared that both heads had moved around on their own--it was during mock-up when I was trying to get the cylinder head studs fitted (which was yet another clue that the heads had moved around previously)--
While fitting the heads just prior to installing them, I had the passenger side head on the deck without a gasket to verify piston to chamber clearance for the final time before installing them for good. I noticed the head was rocking ever so slightly on the deck--I checked to make sure it wasn't binding on the head dowels, and it wasn't. I removed the head and grabbed one of the 990 heads from the LS7 project and sat it on the deck and there was no rock.
Mike dropped by after he closed up shop and took a look at it with me, and wound up taking both heads back with him so he could check them for flatness at his shop the next morning.
As it turns out, the driver's side head was off by .005", and the passenger side head was off by .007". He cut both heads .010" which got them flat again, and the rock was no longer present.
Before anyone jumps to any conclusions about faulty machine work, the person who did these heads for me is hands-down one of the best cylinder head specialists in the country. I know for certain that both heads were flat when I got them back from him as there was no signs of rock during the initial mock-up, they didn't start moving around until after I had assembled them.
Given the number of issues we've had trying to play whack-a-mole resolving all the problems we've encountered with these heads, I made an executive decision--I was not going to proceed any further with this build with them. We've already spent a small fortune on them, not to mention the rest of this engine, and I was not going to risk these heads failing yet again--this time in catastrophic fashion while on the dyno.
I spoke with the owner and discussed the situation with them, and they were in complete agreement with me, so the hunt was on for a suitable pair of replacement heads.
As fate would have it, the [EDIT] 074 heads shown in my pics here turned up immediately after our conversation. They are both 100% NOS and completely unmolested.
If we're going to do this build using factory aluminum heads, these would be our best hope at success rather than taking a chance on another used pair of castings that may or may not be hiding another myriad of issues.
I just took delivery of them today, and tomorrow they are headed to my cylinder head guy to be fully race prepped the same as our current pair of heads.
If there's one thing I've learned in the nearly 5 decades I've been building engines, it's that sometimes no matter what you do nor how matter hard you try, at times like this you have to listen to your instincts and be willing to pull the plug when too many issues start to pile up on top of each other. This is one of those cases.
I absolutely HATE that we spent all this time, $ and effort trying to save these heads to no avail, but I flat-out will not take another chance on these heads.
Hopefully I'll get the new heads back relatively soon, my cylinder head guy assured me he could get these done in a timely manner.
So...until then, we wait.
Stay tooned, more...eventually.
