replacing a piston

If its a choice between a car that runs a little rough or no car at all, by all means go ahead and replace one piston. To minimize any difference in compression, you could re-ring all 4 cylinders at the same time (and even carefully hone the bores after stuffing bunches of rags down to cover the crank and catch the grinding debris). To reduce the chance of imbalance, weight-match the replacement piston to the one that was damaged. Use OEM replacement pistons, pick one that is as close to the weight of the original as possible but slightly HEAVIER, and then carefully grind material off the balancing pad (a machine shop would know where to remove material) until the new piston weighs the same as the original.

Yeah, its FAR better to just rebuild the engine, but when money's tight you do what keeps it running.

Note: this ONLY saves money if you do the work yourself. If you pay a garage to do the work, the'll charge the same to do this as overhaul the whole thing because its not economically feasible for them to take the time to do an in-chassis non-standard job like this.

"pjy" wrote

Personally, on your budget, I would simply get a second piston, and re-use the original rings. I doubt that you will notice any difference. We occasionally install only one piston when doing warranty type work (GM only will replace the failed or damaged part) and it doesn't make any difference in the running of the engine. We also re-use old rings (GM will not pay for new rings under warranty) on specific piston replacements, and it doesn't seem to affect anything.

Ian

Thanks lads. Some very helpful information. I will either fit the old rings to a new piston or re-ring all 4.

Thanks. Yes it is a lot of work. The reason the car isn't already at the knackers is because it's basically sound and I had a new clutch fitted just before the unfortunate mishap. Also I regard it as a challenge and a chance to learn. Valves were damaged but getting replacements is not a problem. I'm just planning on getting it moving again....that would be a major success. After that, the hotrod........not. Your comments much appreciated.

I just work on my own vehicles so have had limited experience getting into the motor (rebuilt 1 Ford V-8 and had to pull heads on 2 other occasions) but I think I spent more than £50 on gaskets. If the budget it that tight and the piston didn't have a hole in it, I would file off any raised metal on the top of the piston (to eliminate a possible hot spot) and put it back together. If piston is cracked or has a hole then I would do as you plan... Maybe someone has some experience with a dent in a piston but I wouldn't expect that to be a problem if driven normally.

The most extreme situation that I have experience with is a stuck valve putting a 1" hole in a piston in a race motor. There was an expert Tig welder at the track, he welded up the hole, we replaced the head with a fresh one and finished the weekend. Here's my take: if this is a cast piston with a hole or an actual dent, change it. If there is just a nick, don't worry too much. If the damage is near the ring land, it's very possible for the ring land to have closed up and the ring is siezed, and compression will go away.

Brian

Thanks for the advice guys. I've managed to pick up a s/h piston and head for £75 and fitted the old rings, connecting rod & gudgeon pin etc to the replacement piston.- made sure nothing was upside down or back to front. Havn't put it back in yet so the crunch ( hopefully not literally ) is yet to come. This is all new to me so thanks again for the info.

Be sure you install the rings correctly (don't line all the gaps up one above the other). Just a detail I thought you should be aware of. An engine service manual for the car may actually give specific angles that each ring gap should be at, but failing that just space the gaps equally around the circumference of the piston.

pjy wrote:

Thanks Steve. The manual just says space them at 120 degrees apart - I'll make sure I do that.