Piston and Cylinder damage - What are the options?

[ianvan]

I have an 3.0 L SC track car. I blew a cylinder ring which has written-off number 6 piston and scoured the bore of the cylinder. I'm talking to my mechanic about options for repair but thought I'd get some second opinions. BTW - I'm new to the engine rebuilding game.
Engine was completely rebuilt 2 years ago with new JE 10.5:1 pistons and the other 5 cylinders (and everything else) are fine.

Scouring is not too bad and may be salvageable by Nikasil replating, I'm told. Is it worth exploring repairing the cylinder? Or is that a risk? Do I need to replate all 6 or can I just do one?

Can I replace one piston or do I need to buy a whole set? Or a pair? I guess sets are individually balanced.

Should I just get a new set of Mahle P&C's and be done with it? That's fairly decent $$'s.

I'm happy to spend the money to do this right and don't want to significantly increase the risk of a bigger catastrophe down the line. But if replacing a couple of pistons and recoating a bore is a robust solution then I don't want to throw money away either. With a track car, there is always something else to spend money on!

Thanks!

[Flat6pac]

Seems to me that JE should be able to match the piston as it would be a standard application.

[Grady Clay]

ianvan,

Why did the ring fail?

I would measure the other five.
What are the piston-to-cylinder clearances?
What are the ring-to-piston land clearances?
What are the ring end-gaps?
Is there any sign of detonation?
(Even a bit of detonation several events ago can have broken a ring and things progressed downhill since.)
What are the piston-to-head and valve-to-piston clearances?
Any sign of contact or ‘near miss’?

How do the bearings look?
Any sign of fracturing from detonation?
Are any of the rods out-of-spec (like #6)?

The #6 ring probably didn’t just go racing one day and say: “I’m tired. I think I’ll fall apart.”
Something preceded the failure.

Broken rings can have many causes.
After two years you tend to look for something wear induced.
With this combination, too often the installation has too much piston-to-cylinder clearance.
After two years wear the piston ‘rocks’ in the bore, the ring lands are worn and the poorly supported ring fails.

I recommend you discuss your measurements with JE.
He can direct your course appropriately.

Yes, he can build a matching piston and you can have the cylinder re-plated to match.
Critical are the other five.
What if they are just ready to say: “I’m too worn. I think I’ll fall apart.”


I have Mahle race P&Cs in my engines.
When I built them there weren’t any (remotely) suitable aftermarket P&Cs available and no one was re-plating cylinders.

While expensive, the Mahle parts are great performers and very long lived.
Today, JE and plated cylinders are a suitable alternative.
Another excellent alternative are the LN Nickies parts.

A factor in your decision is compression ratio.
The Mahle have limited choices of CR but you can get 10.5:1.
The other manufacturers allow you to explore the mechanical limits of CR in a 911 engine.

Sorry for your distress.
Becoming your own ‘best expert’ really pays off.

Best,
Grady

[ianvan]

Thanks for the wise words Grady. Part of why I asked these questions is that my mechanic has suggested either buying a whole new JE piston set and replating all cylinders or buying a new Mahle P&C set. Preferably the latter. He builds great cars and doesn't recommend things unless he has a good reason. With track cars in particular he is pretty conservative and doesn't like cutting any corners. But sometimes I don't immediately get why he suggests things. From your comments I can see why just replacing the damaged piston and cylinder could be dangerous.
I am trying to become an "expert". Its gonna take a few years though.
Incidentally, if I go with mahle's I will use 9.8:1's as the class I race in limits hp anyway and this compression ratio will get me to the 203 hp limit.
Cheers
Ian

[Alan L]

Grady has given you plenty of good stuff to ponder. As he says - critical is whether fixing the one cylinder is going to do a good job, or was it just the first of 6 to take a rest.
You really need to do some exploration on the remaining 5, and try and convince yourself as to why #6 failed. When you confidently have that information making the appropriate decision and cost commitment will be easy. The reason you are asking the original Q is because that info is lacking. Only someone who really knows and has a close look at all the P/cs will be able to give you a good answer. Simple answer - throw money at it and do all 6. But your reservations are valid. So you need an informed answer from a close inspection. Are the other 5 all in good spec? was there something odd about #6? Your engine buider sounds competent. I suggest taking the other 5 P&Cs to him to inspect. But an opinion is not enough. He should be able to give you data/measurements and good reason to repalce them all.
alan

[gsjohnson]

Same thing happen to me and I felt that it was prudent to go thru the whole motor while I was in there. Cheap insurance...
As far as the PC damage was concerned, I found a nice set of six used P/Cs, and I rehoned and reringed them in the interest of cost so I could use the savings on a complete rebuild.

[Henry Schmidt]

JE marks their pistons with a job number. If the piston is what JE calls a shelf piston you can buy one.
95 mm cylinder in good / great condition are available on the open market. We may even have a single to match your set.
Of course new is best but in spec parts are just that, in spec and usable.
Racing is a great fun and you always have something to spend money on so if your other 5 P&Cs are undead in spec, use them and buy a new set of tyres or two with the savings.

[ianvan]

Thanks for the comments. I have done a lot of research since the original post. The upshot is, a new set of 9.8:1 Mahle P&Cs. The JE pistons I had were no longer available having been superseded by a new more robust forging. So, since I could not buy a single piston the least I could do was a new set of JEs and we decided we should replate the barrels as well. The price difference was not big enough to dissuade me from going for all new Mahles, especially since I could only go to 204 hp (to stay in my class) so 10.5:1 CR is not needed. With 10.5:1 pistons the engine would pull 230 hp with out detuning.