V-12 Engine into 308 Build, over on Grassroots

Going to sound crazy but.... Rod stretch? I don't recall the rods you're running.. what's the weight Delta between the pistons normally on the rods vs the ones you're running?
I know.. Numbers numbers..

It's an interesting problem. Either you've got the cam timing way out, can't imagine you'd be that off, or theres more going on with the thermal and structural stresses

 

I'm not sure i understand the question....the clearance I just measured in 0.045" and if i torqued the head it would probably be closed to 0.040. I had targeted 0.050", so not right but hitting at modest rpm/temp? I just don't understand why. .....

 

What are the spring pressures, seat and over the nose

 

Yeah, that's what I was getting at, the closest they come to each other lest they kiss.
Ok, rule of thumb, I don't like getting below these values:
Intake 0.080"
Exhaust 0.100"
Thermal expansion and stretch of the rods.
0.040" that's really tight.

 

Thinking a bit more.. wiggle and rock. The intake valve is wiggling in it's guide and the head will be obviously wiggling around, the piston is also going to have rock. Those two combined will eat away those clearances.. if/when they touch those motions are very prone to breaking the head.. As we've seen.

Just my thoughts/guesses..

 

Just a touch thicker head gasket will give a bit more clearance, but you'll of course lose a bit of compression

 

Seat is about 55#, Intake nose about 185#, exhaust nose about 170#.

 

Right, a quick google and out pops .080/.100 are the recommended clearance numbers. Point. But that can't be right for every engine ever made and when I dig a little deeper and I start to find what I'd expect....smaller engines run small clearances but no real formulas I've found. Motorcycle guys (4v sport bike type builds) are more like 050/080 which is where I got my targets from....a target I missed it seems.

My piston to head clearance is 0.035" with a 0.050" compress thickness head gasket....or it was, I shimmed the liners so its probably a touch more now? That is a pretty hard stop on the piston up at/near TDC and no evidence of contact there but that could eat up most of my valve to piston clearance.

Thermal stuff...the block expands more than the rod so piston clearance should increase. The head heating moves the cam away from the piston. The valves heating are all that's left and is they expand more than lash that would show up as lost hp and poor hot leakdown and doing some math I see where the recommended lash numbers come from with the goal to avoid 0 bucket to cam clearance.

Clearly something is wrong and just adding a lot more clearance would probably fix it, I just wish I understood what exactly is happening.