How to fix mild pre-detonation on forged pistons.
 

How to fix mild pre-detonation on forged pistons.

The subject pistons are Wiseco pn 6595M85
+14 cc dish pistons w/ H.D wrist pins
03-07 Evo VIII

The pistons came into my shop with slight damage around the thin wall of the eyebrow on the outer edge.
They were hung on Eagle rods.
To me it looked like hot spotting and pre-det.

https://www.miataturbo.net/attachmen...ine=1333986908

Notice the damage on the outer edge of the piston?

https://www.miataturbo.net/attachmen...ine=1333986908

the first step is to remove the piston from the rod.
That is done by removing the C clips from the wrist pin bores.
Be careful not to bend or deform the clips, or you will have to order a new set form the piston manufacture.

https://www.miataturbo.net/attachmen...ine=1333986908

Next is to mic the piston to make sure it is within spec.
https://www.miataturbo.net/attachmen...ine=1333986908

Next using a small burr I ground away the damaged aluminum

https://www.miataturbo.net/attachmen...ine=1333986908

After that I used a flapper wheel to smooth any rough spots and to remove the galling

https://www.miataturbo.net/attachmen...ine=1333986908

I also removed the sharp edges (90*) that was from the machining of the piston when it was first made to help eliminate and more hot spots.

Next the pistons were glass beaded clean.
https://www.miataturbo.net/attachmen...ine=1333986908

Now to maintain balance, the piston were weight matched within .2 grams
https://www.miataturbo.net/attachmen...ine=1333986908

Next I checked the balance on the rods, starting with the small end.

https://www.miataturbo.net/attachmen...ine=1333986908

After all the small ends were matched.

https://www.miataturbo.net/attachmen...ine=1333986908

I balanced the overall weight of the rod, also keeping the balance with in .2g .

https://www.miataturbo.net/attachmen...ine=1333986908

Now it was time to hang the pistons on the rods.

https://www.miataturbo.net/attachmen...ine=1333986908

I start by installing one of the clips in each piston.

https://www.miataturbo.net/attachmen...ine=1333986908

After that is done, I oil the wrist pin bores in the piston.

https://www.miataturbo.net/attachmen...ine=1333986908
Also oiling the small end of the rod.
https://www.miataturbo.net/attachmen...ine=1333986908

Then oiling the wrist pin
https://www.miataturbo.net/attachmen...ine=1333986908.

Then sliding the wrist pin through the piston and rod, Making sure the piston faces the proper way on the rod.

https://www.miataturbo.net/attachmen...ine=1333986908

The last step is to install the other wrist pin clip and making sure that all clips are seated in their grooves and the rod floats smoothly on the wrist pin.

https://www.miataturbo.net/attachmen...ine=1333986908

so my method of a little sandpaper while they are still installed isn't a good method?

That is a very common pratice, mainly done when valves touch the pistons to remove any high spots and sharp edges.

As long as you get any and all loose metal and grit it will be fine.

Hm. So you reuse the wrist pin circlips in some cases?

--Ian

Yes, As long as they are not damaged or deformed.

Now a sprio lock found in some pistons can not be reused.

Talking about reducing hot spots, there is some work left to be done on mine...
https://www.miataturbo.net/attachmen...1&d=1334005195

Niklas, Yes I would agree a bit of dressing on all the sharp edges would be nice to do, but it looks like some sort of coating on the top of the piston, with this I would contact the piston manufacture and see what they have to say about removing the coating.

Did you look closely at NiklasFalk's picture? Look again. And again. And again.

That's what your pictures need to look like. Otherwise, another quality post, thanks again for this stuff.

When matching the weight of the pistons, where is the best place to remove material?

I have a quality scale, any advice on how to build a jig to correctly weigh the rods? I would prefer to build something instead of buying something.

Miata2fast.... Depending on the machinist doing the balancing, you will be told all sorts of places.
Some say shallow drill the underside of the crown, personally, I want as much crown as I can get.

I tend to polish the flashing and trash off first, or bevel the sharp edges around the inside ot the wrist pin bosses.

If you look up a few pics, you will see the jig I use. You should be able to copy it.

Are those bearings the rod ends are resting on? Could I not just have a knife edged rest for the rod ends?

How about removing just a bit of material from the skirt of the piston to even up the weight?

I had to enlarge the valve pockets slightly, so I know I will need to match the piston weight before I put it all back together. I plan on doing this myself, so any advice is appreciated.

Lol @ Threadstarter having only 74 posts and + 40 props rating.

They are a double stack of small roller bearings, on the big end and the small end rest on.

You can remove any flashing from the skirt, but go on a 45* from outside to in, but DO NOT knife edge it. Just sit and look at the piston and you will find flashing and trash to remove. most of the time with a decent set of pistions, you will remove about .75g at the most, unless you get one that made it in the wrong box, I have had sets before, have 1 piston be 2g or 3g off

Well, I am tryin to fit in around here.....

Does he even have a miata? He mentioned a ford 2.3 something and has shown a bunch of V8s in his engine threads. Maybe one BP block on his machines?

That's the more impressive part.

Curly, You are correct, I do not own a Miata, I stated that in my intro thread.

I came here for some info for a build that a customer, and a board member here is doing.

The oil pump thread, I am keeping an eye on, I hope to dig thu a few engine build threads, and see what pitfalls and little tips I can find.

Just stay out of Failflora's threads unless you want to know how to do everything exactly the wrong way then.

BogusSVO, how much is the average cost for the job you posted?

Any drawbacks to removing the sharp edges other than slightly lowered compression? Why do manufacturers have sharp edges on their pistons anyways?

It' cheaper to make them 80% complete and some customers just throw things together and place the sticker on the front fender (i.e. don't care).
The next time I'll use custom JE pistons instead, 2-3 times the price is nothing compared to the hassle of not getting what you want.

Didn't mean to sound condescending BTW, merely stating how impressive your posts to prop rating, along with the fact that you don't even have a miata. Proof that we're very accepting of anyone that has something to bring to the table.

Thanks! Another great post... and nice legs :naughty: