Don Terrill’s Speed-Talk

Different brand or style of valve stem seal, or even same brand different material spec as was used 4 years ago??.

Same question about guides/guideliners?

Different valve stem surface material/finish/coating than 4 years ago??

Sure sounds like a heat/surface treatment/lubricity issue.

Would be great to see a pic of the stem when removed and uncleaned, then after being cleaned with only solvent to remove oil trace.
Pic of seals, pic of un bent stems , clearance on un bent stems etc.
I have had a couple of similar problems come in. One in an iron head was being caused by a lapped over guide k liner, the other in an ally head was caused by a cracked guide insert- cracked in the exhaust port end.

Years ago I knew a guy the did not run Ex valve seals on 1,3,5,7 on a on a 350 dirt sprint car. Not sure how he got there but he had a theory the Ex valve stem and guides were not always getting enough oil on that side in some situations like they needed.

It does not bend the pushrod when it happens. Everytime we pull the valve cover the pushrod will still be sitting in the hole correctly. Even the lifter of the effected hole still looks new. Which we have replaced the lifters of the effected hole Everytime. Camshaft still looks brand new. I wondered about heat myself took header off and ran a bore scope through all pipes. There all clear and there has been no change in exhaust setup for years. Changed carbs thought maybe lean condition but no effect. Brand New set of dart heads same problem. I'm not sure on the valve to Piston clearance it's odd this started after a freshen job

What does this guy prelube the stems with?
Still want to see pics if possible.

I'm not sure on that. I will get pictures when I can. I was wondering myself if they had to deck the block again or clean it up this winter and tolerances are to tight after things heat/ expand? It had always been #7 Everytime now it did it on 1 +7 both the same night both on exhaust

Do you run a steam release line from the rear of the heads?
Any cooling system changes?
Different head gaskets?

No steam releases. Nothing new in cooling system. I will check with him on head gasket but I'm fairly certain they are the same

Generally the only thing that bends valves in a running engine is the valve coming in contact with the piston. This is usually caused by not enough clearance in the piston to valve clearance near TDC. Not usual on most SBC's to bend exhaust valves from clearance unless camshaft design is unusual.

The second most common thing that causes the valves to come into contact with the piston is guide clearance or guide oiling. I would take the heads to a good head shop and have them check and set up all the valve guides for proper clearance and seal setup to allow oiling on the exhaust valve guide especially. At the same time they can check all the valve springs for proper spring pressures and make sure there are no weak ones in the mix.

I don't think it's the camshaft unless it is something new and unusual in lift and duration.

The problem is in the heads!

I agree I think it is also in the heads. The very week that he installed the new dart heads it ran that week fine. Must have been a fluke because the following week it bent the #7 valve again. We obviously know it is making contact with the Piston. I'm.not sure on the clearance the engine has but I can find out. He said it's barely even making a scuff on the Piston where it is making contact. So im also thinking the guide must be grabbing the valve.

Camshaft is the same cam we have had 4+ years and he swears it still looks like knew. I know that doesn't always mean it's ok

You need to check piston-to-valve clearance before going any further.

Camshaft drive been inspected? Chain, belt or whatever is not letting camshaft retard is it? Camshaft degreed back in properly after the refresh? Will be interesting to fine out what is causing this. Good luck.

Yes everything has been degreed properly each time.. craziest thing I've ever seen.

Everyone is talking about the cam and degreeing etc etc but if that stuff was not right it would not have run 40 laps before the problem showing up.

I am with the others who are blaming valve guide clearance. Or possibly a different seal than previous starving the guide for oil. You should be able to tell as soon as you pull the bent valve out and look at the stem.