Author Topic: Are my pistons / valves bad? (Read 3704 times)

alim_h · « **on:** July 29, 2012, 08:12:28 PM »

I took the cylinder head off the M42 after doing a compression test:
Cylinder 1: 30
Cylinder 2 - 4: 120+

I figured the valves are bent for Cylinder 1.

The cylinders had markings on them where the valves would be? Is that normal?!

Also, I've attached a pic of the underside of the cylinder head. How would I tell which valves are bent?

Thanks!

MLM · « **Reply #1 on:** July 30, 2012, 01:09:54 AM »

To tell if a valve is bent it should be reasonable obvious that it is not seating correctly. Failing that if you move the cam so the valve protrudes out of the head use a finger to spin the valve and you will see it wobble in a circular orbit. Not spinning true. To get really pedantic do the same with a dti on the valve and measure the run out.

The valve marks on the piston are bad, however dont confuse these with the valve reliefs in the piston top for clearance.

alim_h · « **Reply #2 on:** July 30, 2012, 01:29:30 AM »

Thanks for the reply MLM, but I didn't quite understand - is what is on the piston actually valve marks or are they just the reliefs?

How far do the valves travel when the cam lobe is pushing on the bucket?

MLM · « **Reply #3 on:** July 30, 2012, 03:04:55 AM »

There are valve reliefs on the piston from factory, this is all i can see in your photo. This is correct and required to clear the piston. You can compare the piston in the next bore to see if the piston has any damage.

I dont see any evidence of valve to piston contact. Looks ok to me.

The valve lift is arround 9,5mm iirc

alim_h · « **Reply #4 on:** July 30, 2012, 08:35:14 AM »

I can't see any obvious damage to the valves in cylinder 1 (although I've never seen good valves or bent valves).

Would a machine shop be able to tell me why cylinder 1 has no compression? Could be something with the piston?

kimmerkaze · « **Reply #5 on:** August 01, 2012, 08:22:27 PM »

Could also be your rings. Why don't you compare all the cylinder wall conditions? Maybe you'll find your difference there...

Like MLM mentioned you can turn your camshaft, drop a valve down a bit, and measure runout by rotating the valve. If you don't have a dial indicator just use some straight edge against the side of the valve and visually reference any chances in valve edge to straight edge as you rotate. Compare to another known good cylinder.

kimmerkaze · « **Reply #6 on:** August 01, 2012, 08:23:03 PM »

PS: I'm no expert but this is what I would do if I were you before taking it to a shop.

alim_h · « **Reply #7 on:** August 08, 2012, 01:17:04 AM »

The machine shop was about to seat two new intake valves when they noticed a small crack on the fire ring (?). The guy said the head was useless...

Aaaaaaggggghhhhhhh!!!!

Well, I'll now be looking for a new, used head.