richbobby

I built up my first bp4w using the miataturbo recipe. The engine runs well, makes lots of power, but has always been noisy.

The best I can describe it is a mid frequency rattle at light load, 2500-3500 rpm. It sounds like a washer rattling on a bolt that isn't torqued home. The noise seems to lessen as the engine gets hot. After pouring over every nut and bolt on the engine, pulling the trans to check clutch and related, I began to suspect "piston rattle." The noise does seem to be getting louder over time.

84mm ST 9.0:1 pistons. .0032" wall clearance. Bores were tight on spec throughout depth.
180psi +/- 1psi across the board, and 2.5-3% leakdown
Valvetrain is in spec and has always been right where I left it after an initial adjustment.

I've got 3-4 thousand miles and 100+ pulls, a lot of them at 18psi on e85. If it was a bearing issue, the engine would be toast by now.

Figured i'd have a look at the bores with a scope to see if anything interesting showed- and I feel like this is a bit more wear than I'd expect. You can see the gouging is above the ring max height, and some appear to be deeper than the cross hatch. The pictures are all perpendicular to the wrist pin. It also looks like maybe there is some erosion on the faces of the pistons, but it could just be the shitty borescope. I need to take the time to get each at BDC so I can inspect the piston faces, too.

I'd expect some wear in this location but I'd love some opinions on this. The thing runs great so I'm not so inclined to tear it down, but if I'm going to ruin this block or risk damage it might be wise to pull and change piston brands while I might still be able to get away with a rehone for a 2618 piston.

richbobby

https://www.miataturbo.net/engine-pe...-106581/page2/

this thread feels super relevant. A good read so figured I’d post it here, too.

richbobby

Interesting all the pistons are shiny/clean in the same area I'm seeing the scoring on the bore.

Godless Commie

iTrader: (2)

What is your PTW clearance?
How much ring gap do you have?
Do you warm up the engine before it sees any kind of a load (as opposed to idling for a minute or two, followed by a gentle drive till the pistons have a chance to expand a little)?


Too much PTW clearance will cause piston slap, especially when cold. Piston slap and high loads are not best friends.
Too little ring gap will cause unwanted expansion (if not broken rings), but you have mentioned scoring goes above the ringline, so that is a very remote possibility for you.
As to a cold engine under load, refer to my first sentence.

I could be miles off, tho.

richbobby

P2W is .0032
Rings - .020", .023" .019"

I think I messed up the deck height measurement. The shiny bits on the edges of the pistons may be witness marks of the pistons kissing the head.

As far as warmup- I usually start and let it idle for a min or two, then drive lightly. Letting it idle until coolant is fully warmed up did not have a different affect.

LeoNA

For this application the PTW clearances that I recommend are .004" for 4032 and .005" 2618 alloy. I would check the piston to head clearance, re-hone, clean pistons and re-assemble with new rings.

sn95

iTrader: (2)

Hard to tell from the picture posted but it looks like all of the vertical scapes are in the area of top and second ring piston travel. Who installed and gapped the piston rings? Did the person installing the rings properly deburr each piston ring end after getting the desired end gap clearance? Piston rings that have file fit will often have very sharp edges that need to be debited to prevent scraping the bore vertically.

richbobby

these are about 0.001" bigger than the hivemind suggests- have you run them this "loose"?

@sn95 the machine shop fit the rings and I verified/deburred before install. This isn't a ring issue (see below)

Tore the engine down this weekend. Minimum, worst case (rocked) piston to head clearance measured to .034", average piston to head is around 0.042". Pistons are down the bore ~0.011". Lines up with my initial measurements- though I based my head gasket choice (oem 0.032") on the measurement made at the wrist pin, rather than the worst-case clearance. I thought this was practice, but maybe it makes more sense to do a worst case situation? When the pistons are cold they will rock more in the bore and you'll loose a few thousandths clearance to the head. Could explain why it went away as the car warmed up

No crazy witness marks, a slight discoloration in one of the "shiny" areas.




Possible it was JUST kissing the head, or at least close enough to keep those areas clean.

I'm also convinced that the scratches in the bore are unrelated. I think it is a debris issue, some trash was going through the engine. I noticed a little erosion in the combustion chambers, but cant say for certain if it was there before. I am also suspicious of my turbo- the compressor wheel is super beat up. It looks like it is shedding material.

I found embedded material in the pistons and bearings. You can see a small dot in this piston below.



Scores in the bore are actually not too bad. One or two of them JUST catch a fingernail. One of the crank journals got a bit scratched, may try to diy polish.
I'm going to talk to the machine shop, but I think the plan going forward is a rehone for a set of 2618 pistons, evaluate the crankshaft. New bearings.

also found that my Supertech Stainless intake valves are already wearing at the valve seat. Will prob ditch these for inconel. You can also see the questionable area in the combustion chamber that might be shedding material.

LeoNA

The piston to head clearance should be more than adequate. If the vertical scoring is primarily on the thrust side, it is from not enough PTW clearance. It looks like the crown was seizing. I always run .004"-.005" on these engines. The increased PTW will cause more piston rocking during warm up, but it is much better then seizing a piston under load. This is a compromised design because of the poor rod to stroke ratio and iron block. Have a machine shop hone the cylinders and clean up the pistons lightly with some 400 grit wet paper. New rings, gaskets, bearings and you should be good to go. Make sure you inspect all of the components, oil pump, valvetrain, turbo etc, for other sources of premature wear.

Alejo_NIN

iTrader: (2)

thats how my pistons looked after 50 miles of driving when i spun a rod bearing.
i'm sending them like that.

i'm doing cerakote coating on the skirts tho....super tech pistons as well..never buying those again

LeoNA

What was your piston to wall clearance? The ST pistons are made by Mahle.

richbobby

I sent the machine shop some pictures, and the writer there blamed everything on the tune. Too much timing, too hot EGT. Even blamed the wear on the intake valves on the tune.

I am not a pro tuner, but I am paranoid. Timing on gasoline was based on the turbo kitty map, minus a bit of timing in boost. Timing on e85 was 19* at MBT 200kpa. Maybe a tad aggressive but I was not seeing any knock. I would have figured retarded timing would net higher EGT's, anyways.

afr's were 11:1 or so under boost, 14.5 cruise.

I'm not ruling that out, and I would love to point the finger at myself as obviously something could have been off, but idk.

Alejo_NIN

iTrader: (2)

who fucing knows....i was a noooob..
this time around i handed my whole engine to a shop....i don't have the energy to continue learning by my own mistakes.