Replace piston rings on an 87 Mazda w/blown head?


#1

Musing on a recent show, a caller said he had an 87 Mazda with 190 k miles which had overheated and destroyed the head. He liked the car & wanted to keep it on the road if doing so made sense. He had already purchased a used head for $30 at the local junkyard, and wanted Ray and Tom’s opinion.

Ray said for a $30 risk, no harm to install the head and hope for the best. Could work.

But here’s my question: Ray suggested besides the head, to replace the rings at the same time. Apparently doing this job is doable for a driveway diy’er by removing the oil pan, disconnecting the rods from the crankshaft, and pushing the pistons out the top.

I got to thinking this ring replacing job is going to be a lot of work. Especially when there’s no indication of a problem w/the rings in the first place. Add to that, there’s a risk the $30 used head might not work. Replacing the rings on this car seems overkill. I think I’d just replace the head, and if it worked, drive it. Whatever fails next isn’t likely to be the piston rings.

If I wanted to spend money beyond just the $30 junkyard head, I’d take the junkyard head to a machine shop and have them rebuild that part. And as long as there’s no metal bits in the drained oil, leave the piston rings alone. Might also ask a the machine shop to check the flatness of the block.

What do you think? Does it make sense to replace the piston rings along with the head in this situation?


#2

Two points.
One is to run a dry and wet compression test before disassembly. That test will be pointless on cylinders in which there is a major head gasket breach but very seldom does a head gasket give up on all cylinders. If the unbreached cylinders show a ring problem during the compression test then that could pave the way for re-ringing it. Depending upon two.

Two is that once the head is off the cylinder walls should be inspected closely for scoring and pitting. If cylinders are in that condition then re-ringing would be a waste of time and boring would be the proper option. Boring would mean a complete engine overhaul and not economically viable on an 87 Mazda.
If the cylinders are not severely glazed and if not too out of round at the top of the bore then ringing could work for some degree of time.

Of course if the head gasket breach diluted the engine oil and considering the severity the lower end could be wiped also. There’s a lot of variables here. The use of the word “destroyed” might mean the entire engine should be scrapped.


#3

I’ve been told, and it has been my experience that when you grind or replace the valves and replace the head gasket, the rings start burning oil, even though they didn’t before the head work.

The theory I’ve been given is that restoration of like new compression blows out the worn rings. Maybe, but I suspect that in the final hours or maybe even minutes before the engine failure, the rings were also damaged by coolant or excessive oil, but that damage doesn’t show up until the head goes back on.


#4

I have replaced head gaskets twice and experienced no ring or bearing problems afterward. Both times the head gaskets failed due to over revving.


#5

Would greatly depend on what the bores looked like when the old head came off, assuming a compression or leakdown test was not possible.

You would need a staightedge to check the block and used head for flatness and a hone to rough up the block so the new rings could seal.

I’d pour a bit of kerosene into the ports to check the used head for valve seal. $30 head, $40 head gasket and alotta labor, stupid to just guess at those, right?