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-   -   My First Top End Rebuild - A Few Questions (http://www.gasgasrider.org/forum/showthread.php?t=24301)

sneaky98gt 03-04-2019 01:09 PM

My First Top End Rebuild - A Few Questions
 
Hello all.

First off, I apologize for yet another "what do you think of my piston / cylinder situation". I know there's hundreds of those threads out there, and I've read all of them, but it still feels prudent to make my own, just because it's mine.

Also, I'm a little long-winded; I apologize. I'll summarize my questions in bold at the bottom of the post, if you want to skip to there.

Anyways, as mentioned, this is my first 2-stroke top end...ever. I've rebuilt multiple car engines, but never had a 2-stroke apart before. So forgive my noob-ness.

Bike is a 2011 EC250. The bike ran fantastic before tear down. I've never ridden a 250 with as much grunt as this one has, and haven't yet ridden a 300 that seems to be any better. It really did run well. I run 50:1 Amsoil Interceptor. Jetting is 178 main, 40 pilot, CEK needle on 5th clip. The previous owner had the RB carb mod done, and supposedly the RB head mod; I believe it, as I measured the squish at around 0.8mm before final disassembly.

I've put somewhere around 80-100 hours on it since I bought it; previous owner supposedly put a new piston in and only rode a few hours (< 10) before selling it to me. Not knowing how true that was, I figured it was time to freshen it up and create a known T-0. Foolishly, I did not measure the ring gap, or the compression, before disassembly. Oh well.

Here are some pictures of the piston and cylinder. At the time of pictures, all that had been done is just wiping everything down.

https://i.imgur.com/HDsZpqjh.jpg

https://i.imgur.com/pjej3Dqh.jpg

https://i.imgur.com/VEksmmXh.jpg

https://i.imgur.com/9hplI9ch.jpg

https://i.imgur.com/oPma4OWh.jpg

https://i.imgur.com/4Dxt6gjh.jpg

https://i.imgur.com/wahwsKph.jpg

Honestly, it doesn't look quite as good as I was hoping. There's a bit more carbon on the top of the piston than I was expecting. Also seems there's a good bit of scuffing on both the piston and cylinder, definitely more than I was expecting. That said, I may have had unrealistic expectations based on tearing down car engines.

Good news is that nothing can be felt with a fingernail. Also, at least some of the crosshatching can be seen in most places. And I was surprised that there was virtually no blow-by past the rings.

The other good news is that after doing some research on what these things might should look like, it doesn't seem that anything looks too out of bounds. Thoughts?

I measured everything, which gave mostly good results (I think). The piston measured 66.30mm, compared to the 66.33mm nominal that's stamped on it. So it's worn a decent bit, but I'm replacing it anyway, so not worried. The cylinder measured 66.382mm just above the rings at TDC. It measured 66.385 just below the rings at TDC, and the same just above the ports. Just below the ports measured 66.395. See pictures:

https://i.imgur.com/GzCCoLzh.jpg

https://i.imgur.com/mzHxsgHh.jpg

https://i.imgur.com/YpEEs8oh.jpg

I take all of this to be good news. It certainly seems everything is still well within spec, and there's barely any taper in the cylinder or "lip" above the rings. These measurements also repeated very well around the cylinder, so it doesn't seem egg-shaped at all.

My plan is to clean up the cylinder with some WD-40 and scotch-brite, grab a gasket kit from North Tahoe Motorsports, and to slap a Wossner "A"-piston in it, and let it fly.

Piston: https://wossnerpistons.com/products/8052
Gasket kit: https://www.ebay.com/itm/TOP-END-GAS...4AAOSw6qpa7MXa

Alright, summary of questions:

1. How's the piston and cylinder look? Anything to be concerned about? Just scotch-brite-ing and cleaning the cylinder good enough?

2. The cylinder measures ~66.39 on average; the A-size Wossner posted above (66.33mm) calls for 0.07mm PTW clearance. Is the fact that I'm only at about 0.06mm cause for any concern? I wouldn't guess so.

3. The base gasket is green in color (which usually means 0.30mm thickness), but it's definitely about 0.45mm thick. The N.Tahoe kit looks to have 3 gaskets supplied. Are any of them 0.45mm thick? Or a combination that equals that?

4. Related to #3, but if I have to compromise on gasket thickness, would it be better to increase or decrease the squish? I.e., since I have 0.8mm squish right now, would you go with 0.7mm or 0.9mm if I have to chose via the gasket thickness?


Thanks in advance for the help! This forum rocks!

Jakobi 03-04-2019 05:39 PM

The newer black gaskets with the steel sheet in the middle measure up around 0.5mm when new. They do compress a bit once torqued.

As you've said.. The piston doesn't show any signs of blow by. That's great! But the skirt has definitely seen some action. The burn pattern on top is great.. That is what you want to see on a 2T!

Throw an A piston in it and call it done!

frankinberg 03-04-2019 07:27 PM

I like to use wossner pistons on all my rebuilds I get my kits on eBay from Tahoe north bob great guy

Moto7man 03-04-2019 11:22 PM

Quote:

Originally Posted by frankinberg (Post 188928)
I like to use wossner pistons on all my rebuilds I get my kits on eBay from Tahoe north bob great guy

Definitely go with a Wossner.

sneaky98gt 03-05-2019 06:51 AM

Quote:

Originally Posted by Jakobi (Post 188927)
The newer black gaskets with the steel sheet in the middle measure up around 0.5mm when new. They do compress a bit once torqued.

As you've said.. The piston doesn't show any signs of blow by. That's great! But the skirt has definitely seen some action. The burn pattern on top is great.. That is what you want to see on a 2T!

Throw an A piston in it and call it done!

Why is that burn pattern good? What's a bad pattern look like?

What would cause all the scuffing on the skirt / cylinder? Running the piston too long?

Sorry for all the questions, just trying to learn.

Quote:

Originally Posted by frankinberg (Post 188928)
I like to use wossner pistons on all my rebuilds I get my kits on eBay from Tahoe north bob great guy

Quote:

Originally Posted by Moto7man (Post 188934)
Definitely go with a Wossner.

Yup, the plan is now definitely to go with an "A" Wossner. I'd get the piston from North Tahoe also, but they don't list them anymore. Just the Vertex ones.

john r b 03-05-2019 03:33 PM

piston
 
Shoot Bob an email, Great service and attention.. N.Tahoe powersports
He sell's on e-bay too

RudolfHucker 03-05-2019 03:41 PM

Try this video from Jeff Slavens about piston burn patterns.

https://m.youtube.com/watch?v=SzhWT0HYojs

Jakobi 03-05-2019 04:30 PM

Quote:

Originally Posted by sneaky98gt (Post 188944)
Why is that burn pattern good? What's a bad pattern look like?

What would cause all the scuffing on the skirt / cylinder? Running the piston too long?

Sorry for all the questions, just trying to learn.


Yup, the plan is now definitely to go with an "A" Wossner. I'd get the piston from North Tahoe also, but they don't list them anymore. Just the Vertex ones.

The leaner the jetting, the hotter the piston. Too hot and you blow a hole threw it. Too rich and it stays clean, washed, or spoogey. Ideally you want it hot enough that the oil is burnt off and you get some carbon build up. The clean areas align with the transfer ports where the cooler charges come into the combustion chamber.

Skirt wear is from the piston rocking in the cylinder. Happens due to tolerances, and as tolerances increase so does the rocking. Can make the engine a bit louder, crack skirts, and increase cylinder wear. I usually change piston and rings together around 70hrs. I tend to have some blow by occurring by then as well.

I favour the Wossner pistons too :D

Jakobi 03-05-2019 04:52 PM

Here is a useful link too: http://www.smellofdeath.com/lloydy/p...diag_guide.htm

sneaky98gt 03-05-2019 06:41 PM

Perfect, thanks guys!

Quote:

Originally Posted by Jakobi (Post 188956)
The leaner the jetting, the hotter the piston. Too hot and you blow a hole threw it. Too lean and it stays clean, washed, or spoogey. Ideally you want it hot enough that the oil is burnt off and you get some carbon build up. The clean areas align with the transfer ports where the cooler charges come into the combustion chamber.

Skirt wear is from the piston rocking in the cylinder. Happens due to tolerances, and as tolerances increase so does the rocking. Can make the engine a bit louder, crack skirts, and increase cylinder wear. I usually change piston and rings together around 70hrs. I tend to have some blow by occurring by then as well.

I favour the Wossner pistons too :D

Makes perfect sense. Well then, that pattern tells me what I've always known: the bike is jetted absolutely perfectly, and runs amazingly.

Your comments about the skirt reminded me: I actually took the pipe off and looked at the piston after the very first ride I had on the bike. Here's what it looked like then:

https://i.imgur.com/N1lMfG2h.jpg

Compared to the pictures I took just now (which is at least 75 hours later), it looks almost EXACTLY the same. Since 75 hours of my riding (which while I've never abused it, I'm certainly not easy on it) resulted in virtually no change, this tells me that:

1. The piston has more hours on it than the 10 the PO claimed
2. It has a LOT of hours on it, or the earlier hours were extremely hard

I also just measured the piston again. It's 66.30 directly across the skirt, but if I index around a bit from directly across the middle, I'm seeing like 66.25mm. Again, I'm no expert on what 2-stroke pistons should measure, but 0.08mm from nominal sure seems like quite a bit of wear. Plus that puts the PTW at well over the service limit of 0.01mm, so there's no doubt that it's time for a piston.

The 1st owner had the RB mods done (I'm the 3rd owner). If I was a betting man, I'd say the 1st owner had RB do the head and carb, and re-ringed it at that time. And that the 2nd owner never touched it, and I'm currently holding the original piston.

No big deal. Just interesting. Ordering parts tonight!


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