Ross pistons problem (big one)

[a_sesshoumaru]

Hi, last week i finished my rebuild and i started to use my supra, then two days ago it started sounding like it had rod knock, it souded very bad, so i decided yesterday to take my engine down again to find the problem, crankshaft is fine but then i noticed the pistons have so much play they actually hit the cylinder wall a little, like if they had 400,000miles, i also noticed the pistons skirts looks kind of burn again cause when i boght them used from a member of the forum they looked like that too, so i guess he had the same problem i am having now, i guess that ross are 0.17 oversized, the ring end gap in ring #1 was 0.019 and ring #2 0.027,i still dont know if the machine shop did the job bad or the rings where the problem, but i guess the rings i got the pistons with are useless now, my question is, Can i use 0.20 oversized rings for the ross pistons? or are the ross pistons ring lands different than regular pistons? What ring end gap do you recommend for a 500rwhp? sorry about my english, Help please.....:1zhelp:

 

[Rennat]

I believe thats called piston slap... the piston to cylinder wall clearance was too big, and thus it makes for a "loose" piston... i know IJ or suprarich will have a defintate answer.

 

[NewWestSupras]

I use Ross pistons and they did slap a little when cold. Check your piston to bore numbers. I personally wouldn't go over 4.5 thou. Ensure your bores are round and not oversize. A couple thou makes a huge difference on these numbers, especially when the pistons are cold. Once my engine was warm it would quiet right down. They expand alot. I used 17 and 20 on my end gaps.

 

[DegreE]

Piston slap.

Got it in my Eclipse with the 75shot.

 

[IwantMKIII]

piston slap, odds are your block was bored out of spec causing the noise. Either go with new pistons to fit the correct size or new block bored PROPERLY (given the pistons are okay to use again)

 

[spoolint78]

look at the bore to see if its good.
i know people had problems with 7 thou piston to wall clearance, and the ross pistons would slap.

find out what the piston to wall clearance was.
Chances are, as already mentioned it was too big.

 

[gofastgeorge]

i also noticed the pistons skirts looks kind of burn again cause when i boght them used from a member of the forum they looked like that too, so i guess he had the same problem i am having now,

Were they the set that had a thick coat of moly dry lube on them, but you could still see the scoring that he tried to cover up ?

If so, throw them away, and start over..........

 

[IJ.]

Macro pics of the skirts please.

 

[Slow66]

He bought the pistons from me, and they came out of a perfectly running 550whp 7m. In fact they were inspected (by myself and my machinist) and reinstalled in a freshened motor i was going to run again. I decided to go with a custom piston and sold them to him.

I can 100% guarantee that a failure like this was from improper machining and not due to a piston defect.

If there is anything i can do to help, let me know!

 

[Doward]

Agree with Ian, I want to see the skirts (and bore, if possible) before trying to call the cause here.

 

[turbodriz]

I will just say that slow 66 would'nt sell no bullshit so cancel out the defective piston part. ............ Once again just my 2 cents.

 

[a_sesshoumaru]

i am not saying the pistons is defective but due the oversize of them the machine shop might have done the work bad cause i read they were 0.17, besides i did not metioned any names and i want to use the pistons again, but if the machine shop did the bored 0.20 over can i use 0.20 regular rings? or are Ross ring lands different? i am taking the block tomorrow to the machine shop cause they said they warranty they work, so i have not disarmed the block yet, piston #2 is the one most affected, i want the machine shop to take it apart so they fix it. i will take pictures tomorrow.

 

[a_sesshoumaru]

three pistons were damaged because of the rings, does anyone have 3 good ross pistons 0.17 for sale from another set???

 

[WhtMa71]

Pics of bad pistons?

 

[a_sesshoumaru]

i got my camera stolen two days ago but i will take pictures tomorrow if possible

 

[Adjuster]

I've found if the pistons are coated, and all of mine have been coated, that running the 6k clearance ROSS reccomends results in nasty piston slap.

My current ones are set to just over 4K, and they slap when cold.

I think to get away from the slap, a piston with a longer skirt design would be needed.

Also I think the thermal dispersant on the underside, with the thermal barrier on the top/crown tends to keep heat out of these pistons so well, they do not expand as much as ROSS allows for with the 6k reccomended clearance....

Rings must be filed to fit, wider on the top ring of course.. 17 and 19 gaps are a good rule of thumb. (Really, if you have them at 20 and 22, they would seal up fine.. The minimal amount of blowby lost is nothing in the whole of engine operation.. LOL)

From what I've seen, the Wiesco pistons seem to have the longest skirt design for the Forged piston options on the 7M.. (The Probe ones appeared to have long skirts too.)

Just my .02

Sorry to hear your shop screwed up.

 

[jdub]

In any case, using the same size pistons will likely have the same result if piston-to-wall clearance is too big.