Top-end analysis

[Matt Fisher]

I'm putting a new top end in my 03 CR 250 and thought some analysis of the existing piston/head/cylinder would be good. This is a duplicate of the thread in Mods/Performance/Repair that is now MIA.

Here's the link: http://smg.photobucket.com/albums/v474/MattFisher/

Background:
The last ride was a year ago in the dunes, it was making some minor knocking/rattling noises when I shut her down due to injuring my knee (and the noises). I bought the bike used, and was told it had a new Wiseco piston in there (doesn't look like one to me). At the dunes I was running 2 sizes larger than normal main jet, dropped the clip 2 places, and 1 size larger pilot, flowing 91 octane pump piss mixed with Castor 927 oil.

 

[whenfoxforks-ruled]

Ran lean from a head leak and detonated. My first remark about the rod has been erased, good. But them pits around one side of the crown are curious, anti freeze? And I hope the next piston gets holes drilled on the exhaust bridge, I am pretty sure its included with the directions. The newer pics sure show the exhaust edge going bye bye!

 

[Rich Rohrich]

Now that you can clearly see the erosion of the crown on the exhaust side, I'd have to agree with Blacker's initial thoughts that this is a detonation related. Like Fox I can't really explain the pits in the center without seeing them in person.

That type of damage on the exhaust edge is typical of what you see in heavy load WOT situations like the dunes. I've seen lots of RD350 & TZ250 roadrace pistons that suffered a similar fate.

 

[BLACKeR]

it looks like about everything went wrong with that engine. there's scuffing on both the exhaust and intake side of the cylinder, and the piston seems to have eaten sand, and suffered from detonation. what kind of filter do you run? sand can sift through the foam filters if you dont have an outerwear. also how much warm up time do you do?

 

[whenfoxforks-ruled]

I have seen badly cast aluminum show pockets of dirt like the top of that piston. Something causing the pockets in the aluminum near the surface to open? That piston detonation was in the early stages. Seizure seemed to early, except for the no holes in the skirt. Could just be an attentive rider.

 

[BLACKeR]

also he said he bought this bike used. its very likely all the damage happened before he bought it, and he just happened to catch it now.

 

[Matt Fisher]

No outerware, though I didn't find any sand in the intake boot or carb. I'll have to check the inside foam of the filter tomorrow. It's a DT-1 (aka: Moose). Just donned on me that the OEM filter box arrangement isn't known for it's stellar sealing around the metal ring, I'll check that too.

I idle it until the expansion chamber has enough heat in it to be felt through my glove, then ride casually for a few minutes before dipping into the throttle.

I didn't find any evidence of coolant in the cylinder.

Maybe the fuel I ran was too old, I don't remember? I could have caused the detonation, or it may very well have been from the prior owner. Prior to the last ride I'd been riding it only on MX tracks (slowly), and the only odd noise was the off-throttle clank often attributed to the RC valve slamming shut.

I am wondering why there is zero carbon build-up on the head or piston? I don't recall taking apart a motor before and not finding even a little bit in there. The tiny black specks on top of the piston are hard like rocks, not soft/flaky like carbon build-up.

The new piston will certainly get the skirt holes drilled. Anything in particular that I should to to prep the cylinder for the new piston?



Thanks for the replies, this is fun. Next I'll post some pic of my son's RM65 top-end for analysis. :cool:

 

[BLACKeR]

from the pictures i see it look like the scuffing is just aluminum on the cylinder wall. i would clean that off good. you dont want that on there when you rebuild. you can clean that off with muriatic acid or scrub it off with a scotch pad. (my favorite)

im anal about my bike warm up. i know a lot of folks fire it up scream it for a little and go. i idle with occasional blips till i can feel the cylinder is warm through my gloves. ive never seized a 2-stroke. run the best pump gas you can, and dont let it sit in a plastic jug more than 2 weeks.

 

[DougRoost]

I am wondering why there is zero carbon build-up on the head or piston? I don't recall taking apart a motor before and not finding even a little bit in there. The tiny black specks on top of the piston are hard like rocks, not soft/flaky like carbon build-up.

My thoughts exactly. You didn't accidentally switch fuels with a buddy who rides a 4 stroke did you? What premix and ratio do you run?

 

[Matt Fisher]

Maxima Castor 927 mixed at 40:1. I'm positive I ran pre-mix; I won't associate with those mouth-breathing 4 stroke guys.... Just kidding ;)

If the motor did injest a bit of ocean front property, could that have cleaned off any carbon, or would the detonation have just knocked it loose?

My son's RM65 runs the same fuel as mine, and his piston has a tiny carbon "W" in the center of it. However, I did buy his used too, it could have been there from the prior owner.

So many options, so few definitive answers.

 

[whenfoxforks-ruled]

I gave up on the acid for cleaning. Watch what you are doing and use the scotch pad in parts cleaner or hot dish soap. Or just have a professional lightly hone it ,relieve the exhaust bridge and check that the ports are chamfered. Them blisters on top could still just be casting imperfections. I do not believe the sand caused them, may have had something to do with why the ignition ran advanced though.

 

[Jaybird]

If the motor did injest a bit of ocean front property, could that have cleaned off any carbon, or would the detonation have just knocked it loose?

It sure looks to me that there is a bit more than casting on the top of piston. "target at a turkey shoot" was my first comment from the misplaced threads. (Lol, you need one of these hightech photo gurus to take those shots.)

I would think a bike running bean oil is most certainly gonna leaves signs of it.
Knocking is probably the most violent thing that these engines see. Isn't hard for me to imagine it dislodging carbon. Especially with some silicate to help it along.

Matt, I know you said dunes, so high load is presumed, but were you riding the wiz out of this bike with high revs all the time?

 

[Matt Fisher]

Well, I am fairly confident that the damage was from detonation, not sand. I pulled the filter (No-Toil, not a DT-1 like I thought) and looked for evidence of sand that had gotten around or through the filter. Nothing that I could find. Really not that much sand in the filter either, though that's not a surprise considering I was riding at the end of December so the sand was a bit damp.

What I don't know is if the damage was there before I bought the bike, or if I caused it. I'm going to place my bets on running gas that was a month or so old (?), because I'd been riding it for the last 6 months with no issues.

Yes, I was revving the snot out of it. My riding buddy really likes to do sand drag stuff on his quad, so I'd probably done 50 or so runs, not to mention several 4-5th gear wind-out sessions.

When the Wiseco arrives, I'll check the squish, drill the piston skirt, check the ring gap, and scotchbrite the cylinder. Riding time is not far away, woo-hoo!