Fixing a V-Twin in Thailand
Friday a guy brings an V-twin motorcycle to our garage for a repair, the problem, a blown head gasket on the front cylinder.Now, we have seen this motorcycle before, good looking two-wheeler, but been to all the wrong places. We bored these cylinders for the original owner a few years ago after they were powder coated and the edges diamond cut. All that went along just fine.
Problem happened when it went to the first guy to be put back together, working out from what I can best describe as a modified kitchen. You get what you pay for or sometimes you don't. Anyhow, they managed to assemble the motorcycle again, and in a few kilometers, a piston went south along with a lot of other parts. So that engine was trashed! With no warranty to go back on, off it goes to another repair guy who rebuilt the engine, stuck it back in the frame, and it ran fine... until it blew the first head gasket. Back to the repair guy for new head gaskets. Notice I said head gaskets. Well, here lies the problem. When the engine was rebuilt, copper head gaskets were used instead of composition gaskets. To make it worse yet, the head bolts were tightened to the crazy specs that is in the factory manual, then another turn for extra tightness...
This I figured out when I saw the marker on the head bolts, and the marker on the heads. So anyway, only one gasket was changed on the front leaving the copper gasket on the rear seemingly tightened the same way and in a few kilometers the result was the same.
Motorcycle manufacturers want the cylinder head bolts tightened at the correct torque, and not a extra swing because whatever... So upon removal of the top end, the damage was poking me in the eyeball. The two different gaskets were of different thickness creating an unequal compression situation for one and the resulting damage to the cylinders and the studs is a direct result. Two of the front head bolts were finger tight under the rocker box, so I started to look around.
The two studs on the 2 loose head bolts didn't measure at the right height. Looking closer, I saw that the stud stop shoulders were up from the case a little bit. Not all that uncommon. They do sometimes come loose while taking the head bolts off. So we get the proper tool to set them back to the case. Not a problem, but they did not feel right. As it turns out, they had pulled the threads out of the case, letting the bolts loose. This could have happened from being over-tightened which, in this running load, let the head and the cylinder move, rocking the studs and pulling them up. That could explain the front gasket problem but the back gasket is another story.
The copper head casket used here, was not a full face gasket. This means that the gasket did not cover the entire sealing surface on the head or the cylinder and it too was puked when it came apart. Not the correct gasket for the job. What where those mechanics thinking when they do dumb stuff like this! This kind of mistakes gets people fired at our workshop. If you don't know what you're doing and can't do it right, leave it alone. This guys spent a lot of money on this motorcycle and still has a good amount of problems.
So, on the rear cylinder gasket surface, from the copper gasket, somehow has an impression of the gasket in it that you can clearly see, the impression is deep enough to keep the correct gasket from sealing. Now we have a real problem. If you surface the cylinder deck, you have compression issues, probably also have piston dome/deck height issues, cylinder stud issues, and a load of crap to figure out.
All this could have been avoided if the mechanics stayed within the limits of assembling torque, and used better quality components..
