Replacing a single piston?

'99 Miata engine, I will see if I can post pics of #4 piston later... Let's just say that it isn't all the way there anymore. Assuming that the block checks out, is it acceptable to only replace that piston?

Yes, you can replace one piston, but if you don't pull the engine down and have it balanced to the rest of the rotating assembly, it technically could be out of balance. If you buy the exact same piston you probably won't notice it.

Thinking an OEM replacement. I've considered a full set of overbore pistons, but that's more $ than I planned.

Yup, of course you can !

So long as it balances, and another factory piston will be close enough, sure.

It will be fine. The amount (of weight) it's out of balance will be unimportant, even if it's an aftermarket piston.

Did this in-chassis in a 2000 Chevy K3500 with a 6.5 Diesel.

Dropped the front diff, dropped the oil pan, pulled the turbo, pulled the intake, pulled the injector lines, pulled the head, pulled out one piston and rod, took it to a local shop to have the new rod bushing fit to the rod, honed the bore, cleaned everything good, measured everything, dropped it back in, reused the original rod bearing.

Ran fine.

I can't imagine that being worth the effort when you can get a good replacement engine for $1000.

Is your plan to replace one piston/rings setup, and touch nothing else? Even the TB/water pump/head gasket replacement items will be $300+

TB and water pump done by PO less than 1000 miles ago. Yes, I can get a new (to me) engine for $1000, but unless I see it run and get a leak down, who's to guarantee it won't have issues. Better the problems you know than those that might bite later.

did this on an 86 civic 1.6l. owner liquid-locked it and bent a rod. tried to tell him not to run his cone filter 2 inches from the ground... anyway, we had a blown 1.6 engine in the shop (two cracked pistons, but not the one he bent the rod for...), grabbed the rod and piston we needed, grape honed the bore, swapped em over, and ran it. 310hp @5100 on 24psi with a 50 shot of NAWZ. that was a fun little wedge box...

go for it. itll be fine.

-J0N

I swear I think I remember a friend of mine from the college days (the 70s) replacing one piston in is MGB from the bottom without pulling the head. We did a lot of stuff in those days we wouldn't do today.

Zomby Woof wrote: It will be fine. The amount (of weight) it's out of balance will be unimportant, even if it's an aftermarket piston.

Since when you get into aftermarket pistons, weight, compression height and dome/dish volume greatly vary, I can't agree with that last statement. But get a piston that is close to the weight of the original and you'll be fine.

I'd do a leakdown test on the remaining cylinders first to validate that you've got good valve and ring seals before spending the money on doing one cylinder. Exhaust valves in particular.

I've installed both new and used single pistons in motors before and never had a problem.

bravenrace wrote:

Zomby Woof wrote: It will be fine. The amount (of weight) it's out of balance will be unimportant, even if it's an aftermarket piston.

Since when you get into aftermarket pistons, weight, compression height and dome/dish volume greatly vary, I can't agree with that last statement. But get a piston that is close to the weight of the original and you'll be fine.

Aftermarket stock replacements should be well within acceptable tolerances, but even still, throw a single piston in there of the wrong weight and incorrect dome/dish and it still won't make a noticeable difference. Balance is overrated.

I have repaired Harley motors with different sized cylinders one .040 over the other .060 over the motor doesn't care.

Many racers have built motors with 1 small cylinder to pass tech.

bentwrench wrote: I have repaired Harley motors with different sized cylinders one .040 over the other .060 over the motor doesn't care.

Seems like that might give it a uneven idle.