Sid Wood wrote: > One of my friends at K2W6 has a Revmaster 2300 VW in a Sonex with 70 > hours on the tach. Number 3 cylinder was not making power. After > much trouble shooting and consultation with Revmaster, he took off > the heads. The exhaust valve pushrod was bent. The valve pushrods > are aluminum tubes with steel ends attached...
The pictures of the Revmaster showed two issues, the failed pushrod tip and a bored head that broke into the head studs. The matter of the head stud holes is not a big deal, since those parts of the head don't contribute much material toward the job of sealing the cylinder. See the same situation with my 94mm bored Corvair heads at http://www.n56ml.com/corvair/00120931.jpg , and it's normal for bored Corvair heads. There's just no way to avoid it if you bore it out for 94mm cylinders. I put about 600 hours on those heads with no problems of any kind, including those "breakouts". Admittedly, the Corvair has a lot more material in the sealing area than the VW does. There was also a crack between the valve seats, which is quite common among VW heads with large valves. There's just not much material between the hot exhaust and cold intake, and the difference in properties of aluminum and steel, combined with the minimal material, means a crack between the seats. High head temps of the VW don't help either. Apparently most people just accept it and go on, as the seats don't usually fall out in the VW. 140 HP Corvair heads have the same problem due to their large diameter, which is why only small valve heads are recommended for airplane use. I used Desktop Dyno to run both Corvair valve diameters, and the larger valves don't help at all at the low RPMs we run at in aircraft anyway...in fact they hurt performance up until 3600 rpm or so! I haven't done this for the VW, but the results are likely similar. The pushrods at https://s3.amazonaws.com/expercraft/sidwood/11538091485584b594a9258.jpg look like somebody chamfered the outside diameter of the tube on a bench grinder. I'm curious as to why somebody thought it needed the chamfer. The steel tips are usually chamfered for the inside of the tube. The jagged edges of the chamfer likely started the failure, as you can see multiple sites where the tube failed. And if they chamfered the inside as well, that explains the failure for sure. If the pushrods were cut on a lathe, the ends would be very smooth, and the edges "broken" cleanly on the lathe. This has nothing to do with the above failure, but if the larger diameter aluminum pushrods are used, you're flirting with rubbing against the inside of a pushrod tube and failing it that way (the outside diameter is worn down until it fails in compression). 5/16" 4130 steel pushrods are just about bulletproof, by comparison, and have no issues with rubbing the walls on the pushrod tubes, assuming you ensure that those corrugated pushrod tubes are reasonably straight at installation. Another thing on aircraft engines is that we don't need strong "racing" valve springs. We never get over 3600 rpm anyway. High spring pressures are another way to fail an aluminum pushrod too. For my Corvair, I deliberately went through about 50 old stock valve springs looking for a matched set of the weakest ones to minimize cam and lifter wear. See the middle of http://www.n56ml.com/corvair/phoenix/ for that dirt-simple valve testing setup. There's a lot more on this kind of stuff at http://www.n56ml.com/corvair/valvejob.html, much of which applies to the VW as well. If that's not enough, see http://www.n56ml.com/corvair/ for enough to consume the rest of your month, regarding the Corvair engine. Mark Langford ML at N56ML.com http://www.n56ml.com
