Are these pistons useless?

speed · Postby **speed** » Sun Jun 19, 2016 12:40 pm

I bought a set of rods with pistons.
2 of the pistons are scored as per the photos.
They look worse than they feel, if that makes sense.
I always thought that the compression came from the rings.

Is there any chance that these pistons are use able or would they cause damage to my block?

Thank you!

project.r.racing · Postby **project.r.racing** » Sun Jun 19, 2016 3:55 pm

That will only happen if something was inside the combustion chamber with the pistons. Do you know the history of the engine the pistons came out of?

manga_blue · Postby **manga_blue** » Sun Jun 19, 2016 3:58 pm

Maybe/maybe not. I don't think anyone will be able to tell from photos. They need to accurately measured. Personally I'd take them to an engine reconditioning shop with some competition engine experience or else somewhere like Mania or Deckspeed for an opinion.

TrackAttack · Postby **TrackAttack** » Sun Jun 19, 2016 4:04 pm

It looks like it was caused by knocking/pre ignition...

I wouldn't use them

hks_kansei · Postby **hks_kansei** » Sun Jun 19, 2016 7:45 pm

Can you feel the marks with your fingernail?

If so, they're not worth using.

speed · Postby **speed** » Mon Jun 20, 2016 12:01 am

Thanks PRR, Manga, TrackAttack and HKS.

Unfortunately I don't know the history. Will take them to my engine builder when he does the block and head.

Could just feel it with my nail if I had it in a horizontal groove.

Thank again!

SE · Postby SE » Mon Jun 20, 2016 10:45 am

Yuck My concern would be with that damage near the crown how much other load and damage would have been put on the gudgeon ? You may only ever find out at around 7000rpm

speed · Postby **speed** » Mon Jun 20, 2016 11:40 pm

So where could I purchase another 2 pistons?
I've only seen complete kits online.
Thanks

Postby **NitroDann** » Mon Jun 20, 2016 11:43 pm

Only one reply here so far is accurate.

Take them to an expert who can see, feel and measure them, in person.

Dann

speed · Postby **speed** » Tue Jun 21, 2016 9:34 am

Thanks Dann. I am working on it.
Hopefully this Friday.

TrackAttack · Postby **TrackAttack** » Sun Jun 26, 2016 4:50 pm

I personally believe I have enough experience to suspect the cause, but naturally as this is just a pic, I would certainly suggest getting them checked from an engine shop. What does the top of the pistons look like? Next to no one will sell pistons individually, especially not forged pistons as you want to assemble an engine that can take punishment... Otherwise you wouldn't bother with Forged internals... Do you have the cylinder head that matched the engine these pistons came from? I would expect you will find pitting on both the tops of the pistons and in the matching combustion chambers.

I have seen this exact concern on engines I know were lean at high RPM, from fuel pressure drops... They will probably work on a mildly modified engine, and may last a long time, but if you want something perfectly reliable, and you're paying someone to remove/build/refit the engine, don't skimp on internals, do it once do it right.

As well as all of the above, these 'marks' will remove any responsibility of the person assembling said engine, or tuning said engine. If a piston failed, said person could walk away blaming the existing condition on the failure.

Just my opinion, not saying anyone is wrong here...

speed · Postby **speed** » Mon Jun 27, 2016 12:53 pm

Thanks TrackAttack. Basically I sold an rx7 and the buyer gave me the pistons and rods for a slight reduction in price.
So no, I don't have the head or anything else. Top of the pistons have some carbon build up, so I'm unable to see any pitting.
Will be doing a DIY build but paying for machining through a reputable machinist.
If the engine made 170kw atw reliably I'd be pretty happy.

Cheers!

hks_kansei · Postby **hks_kansei** » Mon Jun 27, 2016 1:26 pm

Talk to the machinist first up, since whatever pistons you use they need them first to measure and hone the block to suit.

Saves the hassle of booking a time and having the machinist see them on the day and say "mate, they're flocked"

speed · Postby **speed** » Mon Jun 27, 2016 2:18 pm

That's the plan. Just haven't been able to get to him before closing time yet. Have tomorrow off, so hopefully I see him then.

TrackAttack · Postby **TrackAttack** » Mon Jun 27, 2016 7:19 pm

I can assure you his/her answer will be don't use them as he/she probably won't (shouldn't) want to bore an engine to suit a damaged piston.

170rwkw is 220ish hp, so the motor will be making more like 240-270hp depending on who you ask. Contrary to popular belief, this is no small feat and engine internals are under a great deal more stress.

The problem with used pistons/rods is you just don't know enough about them. Knock causes damage to piston pin bushes, and these bushes can cost more to replace than the rod is worth... You might come out saving a few hundred on a build that costs a few thousand. Better off spending the extra few hundred IMO

Have the rod bolts been torqued previously correctly? Over tensioning these bolts will cause them to stretch, and if reused, fail. Do you have the 'free length' measurement of these bolts? When a new set of bolts is used, you will record the 'free length' and record each bolts location in the engine. If reassembly is required, an exact amount of permanent stretch can be measured providing an accurate assessment of said fastener. Forged rod failures are almost always fastener related.

What's a new set of piston rings going to cost for these pistons? What bore size are these? They might even be standard bore size which might not work if your engine is a bit on the loose side... So many questions