Broken piston

[SJet]

Well, I just built this big bore 61x motor. I had about 5 hrs on it, and then a chunk of the piston by the exhaust port broke off.

I would like to find out what caused this. Any ideas??

 

[DAG]

oh man i fill your pain.. that SUCKS..!

what do your cylinders look like..

look like the tolerances were to tight and when it got heated the exhaust port had some dinner.. or maby tolerances were good but the exhaust port did not have a clean chamfer...

 

[SJet]

Well, I had .006" clearance, so that should have been fine. Here is a couple of pics of the exhaust port. Maybe there should be more chamfer, but it looked alright??

 

[Mouthfulloflake]

what type of piston is that?

( 006 is too tight from what I remember for wiseco piston. even though its what their instructions say)

 

[sjetrider]

HMMM I think .006 is perfect for Weisco, and .004 for OEM or ProX. Looks like not enough octane or too lean to me?????? Just cause its on exhaust side. Or it could be a WSM piston and it failed???? Post pic of top of other piston.

 

[Mouthfulloflake]

I had an old kawi 750 setup, and it was looser than 006, a few thou more than the recommended ( by wiseco) setup.

it was GOLDEN.

there were threads, were many folks spoke up about giving them more then the recommended clearances too, thats why i always specify that folks who use wisecos need a REALLY competent machine shop.

oh that was YEARS ago though, back on sbtyesterwhocares, so im sure that info doesnt apply to 2007.

 

[crammit442]

Who did the port work on this motor?

 

[keefer]

Lots of deto on that piston before it failed. You need to rethink your compression / octane / timing factors before you rebuild it and do it again. Those exh ports are pretty wide, so good chamfering is a must with all that exposed ring area. You might as well just resleeve that cyl to get it back right.

 

[SJet]

These are Pro X pistons. I thought .006" would be perfect. The other piston is fine. I was running really low comp. It was at 150lbs.

Here's a couple of pics of the other piston.

 

[SJet]

Lots of deto on that piston before it failed. You need to rethink your compression / octane / timing factors before you rebuild it and do it again. Those exh ports are pretty wide, so good chamfering is a must with all that exposed ring area. You might as well just resleeve that cyl to get it back right.

I don't think there was any deto. (See other piston) Comp was 150, 87 octane, timing was 1 degree advanced. Shouldn't this have been alright?

 

[Mouthfulloflake]

if its not deto, its mechanical ingestion right?

are you missing a reed screw?

or was it the bit of ring that broke off from the massive exh port opening?

the pics aren't great on the exhaust port, im sure you could get some valuable input if you had higher quality photos of the damaged pistons actual exhuast port.

 

[SJet]

if its not deto, its mechanical ingestion right?

are you missing a reed screw?

or was it the bit of ring that broke off from the massive exh port opening?

the pics aren't great on the exhaust port, im sure you could get some valuable input if you had higher quality photos of the damaged pistons actual exhuast port.

Yeah, that mess on the top of the piston is the chunk of piston and ring that broke off.

I'll try to get some better pics.

 

[Mark44]

Not much piston wash on that other piston.

Mark44

 

[SJet]

Not much piston wash on that other piston.

Mark44

Well, I guess I could go up one size on the low speed jet.

 

[Big Kahuna]

what is your total setup?? Only time I had this issue was when I ran a speedwerx dry pipe with OEM 38's

 

[ski4]

Well, I guess I could go up one size on the low speed jet.

wouldn't that actually indicate a rich condition?
and still this is really low hrs, i think 5 hrs, would ther be much to see yet ?

im betting a snagged ring, but im for sure in no positon to diagnose anything of this magnitude

 

[D-Roc]

i agree, you may have needed more time for the piston wash to show up and if it did have more time it would look rich, the excess fuel is washing off the carbon.

 

[SJet]

Lots of deto on that piston before it failed. You need to rethink your compression / octane / timing factors before you rebuild it and do it again. Those exh ports are pretty wide, so good chamfering is a must with all that exposed ring area. You might as well just resleeve that cyl to get it back right.

I think I need to put more of a chamfer on the exhaust ports. Why do you say to resleeve?

 

[ski4]

I think I need to put more of a chamfer on the exhaust ports. Why do you say to resleeve?

i think he was trying to save you h/a's and take the easiest route
isn't there a formula or something to figure out what a safe port width is?

does this ex port fall into that ?

 

[SJet]

i think he was trying to save you h/a's and take the easiest route
isn't there a formula or something to figure out what a safe port width is?

does this ex port fall into that ?

Yeah, I think the formula is a max. of 70% of the bore dia. These exhaust ports are under that.