11 replies to this topic

[JonC]

Long time lurker, about 3 years into my resto of a tahiti. Bodywork is at the paintshop at last so onto the engine. It's fully stripped and looking good, will have new mains,big ends and bores measure up good. I've just cleaned up the pistons ready for mew rings but no 2 is giving me grief. The crown has loads and loads of pock marks evenly distributed across the whole face, can see the pistons id numbers at all. All others are perfect.why only this one I wonder, and can I use it? I'm concerned that the pock marks will be a cause of hot spots.

[JonC]

Some pics I hope

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[absx2]

Sometimes when a head gasket is leaking but not enough to be diagnosed quickly water will be drawn into the offending cylinder.

Water in a cylinder is extremely corrosive in the combustion process and will quickly eat away at the piston crown and bore for that matter.

The gasket leak could have been as slight as a misfire from a cold start that clears itself quickly then runs fine.

 

I would advise against new rings in an old bore unless there is absolutely no wear line whatsoever at the top of the bore and the bore is correctly honed.

The top ring will break if there is a ridge no matter how small at the top of the bore.     

[sledgehammer]

Not sure if safe to use - but if in doubt , chuck it out (if replacement is available)

 

I've had an old 2.0 pinto engine do this , it was a plug insulator cracked , tiny bits of ceramic kept sticking to the piston surface , causing a hot spot

 

only found out when the ceramic bit stuck to a valve seat & head taken off due to compression loss

 

I suppose it could be detonation in that cyl if a bit of carbon is glowing , setting off the mixture early on that cyl , or possibly a weak mix due to air leak

 

is there any damage to the ring area's / close by to these areas ?

[ACDodd]

New pistons and drop the compression ratio. Its too high for the spec it was running before.

Ac

[Turbo Phil]

Water in a cylinder is extremely corrosive in the combustion process and will quickly eat away at the piston crown and bore for that matter.

If that were the case how would people safely run water injection to suppress detonation ?

Phil.

[absx2]

 

Water in a cylinder is extremely corrosive in the combustion process and will quickly eat away at the piston crown and bore for that matter.

If that were the case how would people safely run water injection to suppress detonation ?

Phil.

 

 

To be fair phil  I don`t rightly know but I have fixed three minis and about five diesels that had that type of damage so being just one cylinder I thought ( just a guess ) it might be that over detonation but after reading sledgehammers post it made more sense.

We all know water is corrosive in an internal combustion engine and ethanol creates pretty much the same thing and it will wear things out faster but I would not think that is the first priority for anyone running with water injection as its not going to be a high miler.  

On the diesels I fixed there was clear evidence as the heads had cracked into the water jacket in one instance it left dozens of rain drop sized craters in the bore as well as the piston crown, being much higher CR than a petrol engine this may be just a diesel thing ?

AC is spot on as usual with regards to the CR achievable with modern fuel as my little motor is much happier after dropping from 10.3 to 9.7:1. It`s not a fussy car anymore but prior to that it was a lottery at every tank fill as to whether or not it would detonate and or bog down and run on for 20 seconds after i've switched off.

[Turbo Phil]

That's certainly interesting about the diesels. I've never seen or heard of any issues using a controlled water injection system. I guess the difference in a fine atomised mist over a volume of water simply passing into a cylinder would be significant.

Phil.

[JonC]

Ta for replies chaps. Pistons are stock standard size this is the only piston with this damage. I'm leaning towards the leaking head gasket theory, the machine shop noticed a slight breakthrough between 2 & 3 so had a light head skim. New piston it is.

[JonC]

Ta for replies chaps. Pistons are stock standard size this is the only piston with this damage. I'm leaning towards the leaking head gasket theory, the machine shop noticed a slight breakthrough between 2 & 3 so had a light head skim. New piston it is.

[Spider]

It does indeed look to me to be either something foreign or water damage, but not pinging or pinking. Given it's only one cylinder, I doubt it's something bouncing around in there.

 

Phil, I read and agree with what you've said regarding Water Injection, however if the water isn't atomised it will do just this.

 

Damage from Pinking isn't all over the crown of the piston like this has been peppered, but tends to be courser in the damage and more localised to particular areas on the crown, It also hammers the crown down to pinch the top ring and often all rings, breaking the lands too.

[Cooperman]

Make sure you accurately compare the weight of the new piston to the existing ones. Use the small electronic ones available for use in a kitchen. Although the part numbers may be the same it is vital to check.