pistons

[ahistand]

I dont know much about TDI's but a friend has one with the head off, he dropped a valve or something, anyway, it trashed one piston, and of course the head on that cylinder. He's looking to replace just one piston and connecting rod. Some say its not a good idea to replace just one, because the rods are sold as a set, and so are the pistons....meaning they are probably weight-matched. Supposedly there are numbers on the rods that match them together.
So, the question is, can one just buy a good used piston and rod, or should it be matched to the rest? I've rebuilt several 1.6 motors but always replaced the pistons as a set, so I never had to worry about it.

[CoolAirVw]

You can replace just one, but its not the best thing to do.

You take all the pistons and weight them then grind the heavier ones to match the lightest one. If its off by much you cant grind much off. In a perfect world they shouldn't be off more than 1 gram.

.....always replaced the pistons as a set, so I never had to worry about it.

Just because the pistons came as a set dont mean they are balanced within 1 gram. (Mine were though. Kolbenschmidt brand. See <a href="viewtopic.php?t=6550&postdays=0&postord ... e&start=15" target="_blank">THIS THREAD</a> (7 POSTS DOWN)


You cant really balance the rod correctly without special tools. Take all your rods to a machine shop for balance.

Note: I've seen a shop down the street from mine just yank one piston/rod out and drop in another used on without replacing rings or worrying about balance. The car drove just fine afterwards with no vibration issues. How long it lasts??? I dunno!

One other thing. Make sure the piston measures exactly the same as the original.

[caveman]

I yanked one out of an ALH and slapped it into my AHU. Just cleaned up the bore, replaced the bearing and made sure the oil jet was clear. Still running hard after another 80,000 k. Doesn't mean it'll last forever, but if has lasted so far i think it's okay.