Wednesday, March 18, 2015

More connection rod stuff

Looked at the connection rod that broke and the new one Aeroconversion sent. I think I know some more of what can have happened. The rod consists of two pieces, the rod and the cap. The rod and the cap can be made as one single unit or as two units by forging or casting. After the forging or casting, they are machined as one unit. This means that each rod only fit with it's original cap.To prevent interchanging of rods and caps, they are imprinted with (semi) unique numbers. So far so good.

The original VW rods are too large when the stroke is increased to 82 mm. Therefore the rods needs to be modified, as well as the casing. Special long stroke rods are manufactured, but Sonex use ordinary VW rods and grinds them down so they fit. This grinding removes half of the threads, which is a rather odd thing to do (mildly speaking), but a more subtle thing is also happening. The grinding also removes the original unique number on the cap. This can be seen on the new rod I got, which was grinded much less aggressive than the original 4.

The number 929 is printed on the rod, and the same number is almost fully visible on the cap. On the original that is grinded more aggressively, a new number is printed on the cap. They have also used different (larger) fonts.

The number 744 is clearly visible on the rod, but on the cap something has happened. It seems as someone has made a printing error before grinding and then just corrected it. The number on the cap say both 774 and 744. What is clearly visible on the rod/cap (when looking) is the rod and the cap does not go together as one unit. The rod is 744 and the cap is 774. None of my other rods have these numbers.

The picture above shows the new rod with it's correct cap. They fit together like a hand and a glove. The picture below shows the old faulty rod, and it is clearly visible that they are made from different pieces.

Also when measuring the diameter of the main hole, the old rod is oval because the pieces do not fit together. Luckily the threads broke when I torqued it, also because of this mismatch and because of the small thread length. If they hadn't broke, then the engine would probably run for a couple of minutes, and a large bang would destroy the fun. I have to wonder though, what will happen to the poor guy who have received the number 744 cap?

Another thing is also important here. With the new rods I received from Hummel a detailed instruction followed. Here is step number 3.

The info written in large red letters is nowhere to be found in the Aerovee manual, and this is also very strange. Clearly that step is very important.

OK, the connection rod issue is sorted out. Time to move on with the engine. There is still the thing with the heads full of sand and I don't think I will be in a mood to mess around with an aerocarb.

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