-The tz250racing Message Boards
+--Forum: TZ Engine Tech
+---Topic: Piston damage! started by RDJeff
Posted by: RDJeff on July 26 2010,11:40
What do the experts say?
I have about 250 miles on a complete rebuild on my 4DP, and was going to run the top end for one more race weekend, but all my years of racing 125s made me take a peek under the heads first. Good thing I did!
One piston/head has significant damage from what appears to be a foreign object. The squish area is all chewed up/eroded, the ring is stuck at about the 4 o'clock position, where there is a chunk of piston missing from the top down to the ring. From the 2 o'clock position all the around to the 10 o'clock position, the area above the ring is all shiny. The area of the head where it transitions from the squish band to the combustion area has a few deformed areas, which to me look like debris hammered them.
The engine was running fine when I pulled into the pits after the last final. The only problem I've had is a failed power jet solenoid, on this cylinder. I jetted up to compensate, and it appear to be a touch rich if anything, the plug and piston both showing a nice brown color.
I inspected what I could of the bottom end, and all seems fine, no debris in the bottom end, it doesn't appear to be missing any bearing needles or anything. The reeds are all intact. I put new pistons, etc. in both cylinders, buttoned it all up, and did a leak test, and it only dropped 2 psi in an hour, so I think it is tight enough to run.
My theory is that a rock came off the tire of somebody in front of me, and went through the intake and up into the combustion chamber. Once it got in there, it got smashed between the piston and the head, got crushed, then the piston debris and the rock bounced around and got crushed up smaller and smaller, then went out the exhaust.
I'm having trouble posting photos, so can't put any up to look at.
Posted by: anand on July 26 2010,12:39
or maybe bad luck and a piece of piston broke off and caused the bebris damage.
hard to say....just good you found out in time.
Posted by: dand on July 26 2010,2:27
I had some debris problems int eh past, checkout the pics in this post.
< http://www.tz250racing.com/cgi-bin....1;t=453 >
It sounds like you damage was worse though, maybe a bigger pebble/rock.
I ended up putting screening for a screen door, around the intake area (TZ125, no airbox) and I haven't had the problem since.
Dan
Posted by: vince24x on July 26 2010,2:47
we need a pic: from the description it could be debris (from inside or outside) or detonation. ....but a pic will pretty easily tell you which one, generally.
if the reeds are all good, it's hard to imagine something really sizeable from outside to have gotten all the way up top w/o hitting something along the way.
vince
Posted by: RDJeff on July 26 2010,4:40
It wouldn't have to be real big to hammer the squish area. I wish I could post up a pic, can't figure out why, maybe some network protection.
I just took it over to a local two-stroke specialty shop, and they are saying detonation. I'm still not sure...
Posted by: Mike on July 26 2010,7:07
| Quote (RDJeff @ July 26 2010,4:40) |
| It wouldn't have to be real big to hammer the squish area. I wish I could post up a pic, can't figure out why, maybe some network protection. |
u have to watch your file size when posting pics. You'll see the max allowed size when u post an attachment. too big = no post.
check it out.
Posted by: RDJeff on July 26 2010,8:35
Mike, can I e-mail a photo to you for posting? The problem isn't the file size...thanks,
Jeff
Posted by: Mike on July 26 2010,9:57
| Quote (RDJeff @ July 26 2010,8:35) |
| Mike, can I e-mail a photo to you for posting? The problem isn't the file size...thanks, Jeff |
Shouldnt have any probs, others seem to be posting pics... but regardless, yes, please send. mike@...etc
I'll post here.
Posted by: Mike on July 27 2010,11:32
Here are Jeff's piston pics
Posted by: Mike on July 27 2010,11:38
...
Posted by: RS Addict on July 28 2010,12:34
Check to see if the piston ring retaining pin in the piston is still there - it's a bit hard to see from your pics whether it is still intact however, the marks on top of the piston crown make me think it may have dislodged....
Either way I think I lean towards something hard has banged up and down during combustion. If it was the retaining pin - you're lucky you caught when you did before the ring moved and snagged a port......big damage then!
Posted by: RDJeff on July 28 2010,1:11
Thanks Mike for postring the photos!
Addict, the locator pin is still in it's place and intact.
the engine was set up with .027" squish, timing at 1.55mm, running 370/360 main jets, 60 power jets, Trick 110 octane leaded gas, and A747 at 25:1.
This is similar to the way I had my RS125 set up, and I ran it for 8 seasons without ever having a mechanical. this is the first top end for my TZ, and I had this problem!
Posted by: vince24x on July 28 2010,4:00
the pics are a little saturated, but it doesnt look like det....more like something got in there.
detonation is often easily identified by close inspection of the top edge of the hole where they press the ring locating pin in. that edge should be sharp like a razor and in my experience det ALWAYS starts at that location and rounds off the edge...then migrates to the edge of the piston proper.
hope that makes sense.
vince
Posted by: RDJeff on July 28 2010,4:54
Yeah, the pics aren't great, and they don't show the full extent of the damage.
I'm thinking something got in there, did some damage, making hot spots, and that started the detonation cycle. The side electrode on the spark plug even has evidence of physical damage, that you wouldn't expect to see if it was pure detonation. You can see where the ring is pinched, right where there is a big chunk of piston missing, I can't imagine detonation be localized like that, but I've never detonated before.
I double and triple checked the timing, deck height, squish and eveyrthing else I can think of while I was putting in new top ends, so it should be okay. I'll just have to peek inside more often!
Posted by: RDJeff on July 28 2010,4:55
Oh, and screens have been installed between the duct openings and the air boxes, that should help keep rocks out of the carbs.
Posted by: vince24x on July 28 2010,5:11
| Quote (RDJeff @ July 28 2010,1:55) |
| Oh, and screens have been installed between the duct openings and the air boxes, that should help keep rocks out of the carbs. |
another trick (although i dont do this due to the mess): you can line the inside of your airbox w/ grease. this acts like a fly trap to get any debris that gets in there...even smaller stuff. and, it wont affect the airflow like something in the intake might.
hth,
vince
Posted by: RDJeff on July 28 2010,7:09
Yeah, I used to rely on the blowback coating the airbox to take care of the filtering duties, but I found some heavy duty poly window screen with a big mesh. I figure the airbox will get enough air from other places that the screens won't have any effect at all.
Posted by: Tek Shop on July 29 2010,8:47
That doesnt look like debris, it looks like the piston has just let go. Detonation can start it and then then crown just falls apart...Imperfect piston made from an imperfect process..
check your ring pin is still there as well..
Posted by: RDJeff on Aug. 05 2010,4:49
Well, I have new pistons, etc. in both cylinders, screens over the ducts, timing retarded to stock 1.5, a functional power jet in the previously bad cylinder, and a little extra deck height and squish clearance on that cylinder. We'll see how it goes this weekend. Wish me luck!
Posted by: vince24x on Aug. 06 2010,1:10
good luck!
vince
Posted by: vince24x on Aug. 06 2010,1:11
ps: how far is it from AK down to portland? any chance we can get another 250 on the gpru grid this month???
vince
update: ok, i just looked at a map; you are excused! man, AK is up there!!
Posted by: RDJeff on Aug. 09 2010,11:29
Trust me, I'd love to give it a shot, but all the organizations require a school just to qualify you to get on the track, and unless you're made of money, it's a logistical and financial nightmare!
Posted by: SZR on Aug. 09 2010,8:46
I've seen a couple of cases recently where some near new 5KE pistons have broken it's crown a piece about 1/8 of it's crown starting from one side of where the ring pin locating dowel is. It's as if the ring has dropped into the port and grabbed breaking taking the crown with it, however both cases show no sign of ring catching the port.
No detonation signs on the piston, nothing foreign, nothing obvious. Just like a bad batch of 5KE pistons.
Anyone else seen this lately?
Posted by: vince24x on Aug. 10 2010,12:35
| Quote (SZR @ Aug. 09 2010,5:46) |
| Anyone else seen this lately? |
haven't seen this, but it sounds ominous!
vince
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