head cracks

[marlinz]

I just pulled my heads because I stripped a spark plug thread on one side. I thought it might be less messy to install an insert with the head removed. (no shavings etc. in the cylinder). I decided to pull them both so I could inspect- maybe lap valves etc. I noticed that all of the cylinders have a crack in the head between the valves. the valve seats do not appear to be cracked. Compression check showed both front cylinders 79/80 and both back ones 64/80. Are my heads trash?

[gammaxy]

I think that is typical with these heads. If you replaced the heads and then checked them 10 hours later (maybe sooner?) they'd likely be cracked in the same location again. My understanding is the crack doesn't go deep, just through that tiny bit of metal between the valve seats. I've noticed some heads remove the little bit of aluminum between the valve seats where these crack--I assume they don't get these cracks.

Mind sharing how you stripped the spark plug? I'd really like to avoid doing that if I can :-)

[marlinz]

I am not sure. Maybe overtightened last time? When I turned it to remove it, it loosened on the first turn like it should, but then it turned harder after that. After several more turns it went really hard so I tried to turn it in a little, but no improvement. So I just wrenched on it and brought the rest of the threads out with it. there was about two threads close to the end of the plug that looked like the aluminum was welded on, but it was possibly just from the galling that happened from overtightening. I read on here that you can easily overtighten the plugs. Myself, I am going to make sure that I use anti seize every time and be really careful how I tighten them. ???

[kmacht]

Very typical place for the head to crack. The other common area is between the valve and the spark plug hole. Between the valves is usually benign and can either be left alone or welded up. A crack to the sparkplug hole tends to keep propogating and the head needs to be replaced. The risk for cracks between the valves is that if the crack goes too deep the valve seat could eventually loosen up and fall out. If it were me I would have it welded if it looks like more than a hairline crack. Of couse I also suggest checking with Sonex for their opinion.

Keith

[Area 51%]

The crack not withstanding, the most common reason for a spark plug to strip an aluminum head is a build-up of carbon on exposed (in the combustion chamber) spark plug threads.
Anti-sieze is always your friend with dissimilar metals.

[marlinz]

Is it normal to have some soot build up in the intake ports? Would that be a result from not having the valves set right? The engine has about 60 hrs on it, and the clearance gets less between adjustments or doesn't have any clearance at all. That would keep the valves from closing all the way.

[rizzz]

The late Bob Hoover posted something on his blog about the cracked VW heads years ago, worth a read:
http://bobhooversblog.blogspot.com.au/2 ... -head.html

As always, take this with a pinch of salt. Bob Hoover is considered by many as a VW legend, but certainly not everyone, he has/had many opponents as well.

[gammaxy]

Is it normal to have some soot build up in the intake ports? Would that be a result from not having the valves set right? The engine has about 60 hrs on it, and the clearance gets less between adjustments or doesn't have any clearance at all. That would keep the valves from closing all the way.

Mine needed adjustment at every check also--to the point I ended up having to make shorter pushrods. Does the torque on the head nuts also need frequent adjustment? I've wondered if the frequent adjustment is due to the copper head gaskets crushing. When I rebuilt my engine (with new copper gaskets) I was very careful with the torques and re-checked them a few days in a row while it was in my workshop. Anecdotally, the second build seems to have more stable clearances, but there's always the possibility that the engine had already "stabilized" by that point.

I've had both heads off and checked for valve leaks with alcohol and re-lapped 3-4 valves. Like you, I'm curious if the "soot" build up on the valve seats causes the leak or is a symptom. I'm a little suspicious whether the lead in the fuel causes the problem. I'm also curious to hear if anyone carefully lapped their valves before they first flew their engine and how long they had good compression for.

As far as the cracks between the valve seats, I've decided to keep tabs on them and will become concerned if I see them extend below the valve seats (I am unaware of any Aerovee owners who have seen the crack extend to that point). This opinion of mine is largely due to noticing that other similar heads simply have a gap between the seats in this area.