Exhaust valves hitting pistons
We have a 598ci that the pistons hit the exhaust valves only in cylinders number 3 and 7. All the other piston and vlaves are OK.
Brief history:
The heads are Dart Big M 410s. Chapman racing heads did all the work.
We use TD rocker shaft assemblies. Smith Bros. push rods, .100 walls.
We used these heads on the 565CI we had before we built the 598. NO Problems.
About a year ago the engine blew the head gaskets into the valley on
cylinders number 3 and 7. At that time, you could see that the pistons and exhaust valves where touching. The head needed resurfacing and the chambers touched up to clearance the pistons again after the resurface. We thought that this would clean up the problem. We ran it thinking all
was ok to the end of the season in November. When we disassembled
it to go through it for this year we noticed that the pistons and exhaust valves in cylinders 3 and 7 had been contacting each other again. As we had approx 100 runs on the engine, we thought that the valve springs were getting weak and did not think anymore about it because we change the valve springs, exhaust valves, pistons and bearings at about 100 runs. Again Chapman Racing Heads did the work on the heads. We mocked
it up and checked all the clearances as always.
We took it out for test and tune about 3 weeks ago and put 2 runs on it, it ran fine, so we put it away until the first T/S race at our local track. In the ninth run, it ate number 7 exhaust valve. After we took the head off to inspect the damage, we seen that number 3 exhaust valve has been touch the piston again. We again measured everything and we can see no reason for this to be happening. We are lost for a explanination for this to keep happening. And it is only on cylinders 3 and 7.
Spec info:
Crane cam sold roller/Crane roller lifters .810 lift int, .800 lift exh. timing 31°BTDC @.050".
Triple springs rated at 908 pounds at full open.
TD rocker shaft assemblies with 1.7 to 1 ratio
JE Pistons/46cc domes.
.005 deck clearance
Dart heads with 118cc chambers.
Any ideas or help would be appreciated on what is wrong. We can't afford to keep rebuilding this engine and
until we find out what is causing this, there is no sense trying to run this engine.