Technical Valve Stuck in Guide?!?!

Ok, I have been messing with Chevy motors for a long time and had a new one on me today. I am working on a BBC with an 871 blower and had an exhaust valve hang up in a guide (open) after only a couple hours run time or so. This motor is fresh, heads just came from the machine shop, etc. Valve springs are good to right at .650" lift so not necessarily lightweight springs. Motor was running like a champ right up until it started sounding funny. It has open exhaust, so isn't the easiest to hear little issues, but you could tell something wasn't right. Anyway, I had cylinder #3 go dead on me and pulled the valve cover to see what was wrong. I found the exhaust rocker on #3 cylinder off, the push rod in a bind and bent, and the valve hung open. Got the head pulled off today and the only real issue is that it seems the stem on the valve is pretty galled up and it was hung open which then caused it to smack the piston lightly bending the valve. No other signs of issue and the poly lock was even still tight on the rocker stud (even though I convinced myself that if it did come loose, it wouldn't have caused the issue I found here.) Anyway, anyone else had a valve hang up in a guide for no apparent reason? I can give more information on the whole setup if that helps anyone, but it just seems strange to me considering everything is fairly new on this deal...

I do have another set of heads that are fresh and I will likely just swap them over to get going quick. But, I would also like to explain why this hung up when everything seemed "right" going together.

 

Not enough guide clearance

 

Usually it's lack of clearance or lack of lube when a valve galls in the guide. A bit of grit could do it as well. Did the shop assemble the heads? I like to do my own so I can feel clearances.....

 

That was my first thought, but my machinist that built the heads has been doing this for a long time and I wouldn't have thought would miss that. I hope that is it.

 

Yes, my buddy that has been a machinist for probably 40 years assembled the heads other than me swapping the springs after they were assembled and on the engine.

 

EVERYONE...makes mistakes...sooner or later !
Three reasons a valve will hang up in a fresh engine, in order of probability.
1 - Too small a valve to guide clearance.
2 - Not enough quality oil on the valve stem / guide surface during the initial start.
3 - Bent valve.

Mike

 

could it be the piston kissed the valve first bending it and causing the galling instead of the other way around?

 

I am thinking you are right and he just didn't get this one set up right from the beginning. These heads were actually brand new from GM with probably only a few hours of run time even before he went through them. So, technically they could have been a tight fit from the factory or not enough oil on them.

I wondered that same thing and it is possible, but it just seems that the most likely scenario is that it galled making it hang open first. But, I have been wrong lots of times before too...

 

Could there be ethanol involved here ? Soon after it came on the market, Chevrolet & many others recommended a minimum .002 clearance on both intake & exhaust stem to guide ....

 

Nope, haven't run ethanol in it that I know of but good question and information.

 

With the current quality of parts, it is possible that the valve stem itself may have a spot that was oversized. Something that might not show up until the heat builds a few times. With the gulling, it would be very hard to determine that was the problem, but it is another possibility to add to the list.

 

Might be a good idea to clay the piston on an adjacent cylinder to help rule out the bent valve theory, or at least do it on the next set of heads to verify valve to piston clearance. The exhaust valves can grow quite a bit.

 

Yea, good idea. I did pull the other head as well and this is the only valve that contacted a piston. But, I would like to rule everything out as best I can when it goes back together.

 

yes,
..it was an exhaust valve..
any signs of running lean?

 

A coworker had a fairly serious low hours 340 self destruct last year, new edelbrock heads and all the guides were super tight, finally one stuck and in the end the crank was reused…

 

Actually appears it was running rich.

 

Doh! That hurt I bet.

 

Not me just ask one of my ex wife’s! lol

 

could excessively rich wash a valve stem of lube causing metal on metal?

 

I've experienced this twice; the first one the machine shop screwed up and got the guide/stem clearance too tight. The second one the machine shop screwed up and didn't clean the guide well enough before installing the valve. Different shops different types of engines and years apart...

 

All good info guys! Thanks!!!

For reference, here is the galling on the valve stem.

 

Polish the valve stems length ways with 600 grit wet or dry paper. The ground finish on the stems is not that smooth, a finger nail will feel it. Doesn't take much to take off the high spots like when platform [?] honing cylinders. Of course the necessary clearance and lube is important. In the pic just below the galled area you can see the cylindrical grind marks.

 

That looks like a hell of a lot of material taken out of the stem from a lot of debris. Maybe it's just the picture. You mentioned new factory GM heads so I'm guessing powdered metal/iron guides. Maybe the guide was fractured then disintegrated and produced the debris to cause that.

Your engine guy may have recorded the stem clearances if he measured them. Worth asking.

 

Yeah…there was pieces of cylinder wall in the intake….

 

Pull another valve or two out of that head and measure clearance, I bet they are all tight.

 

I'd be willing to bet that S.O.B is bent!

 

Any sort of valve seals being used ?

 

Yep, definitely bent.

Yes, umbrella style seals.

 

I would say that may be able to happen, but this isn’t running excessively rich. It has a top hat style injection on it and not too far off as far as air fuel ratio.

 

If it was hit by the piston first as a cause, there would be no galling, just a bent stem and jammed tight in the guide.
It gradually galled until it scored things up enough to grab it, then the piston tagged it.

I agree, definitely check them all. I bet this valve just happened to go first, but others were nearly there as well..
Supercharged use guide clearance should be at least .0005"-.0008 larger than naturally aspirated, just because of more heat in the cylinder means valve head transfers more heat into the stems.
Hopefully your head guy was aware of this?