Piston Score, Eric Gorr?

[76GMC1500]

My piston in my 87 CR250 has a big ol' score in it centered right over the exhaust port bridge. It also flat spotted the rings a little. I pulled the jug recently, there was no damage. 4 ride later, there was a giant score. 2 things happened differently, I switched to Amsoil, but I also went 4 rides without changing the tranny oil. I read in Gorr's book that a leaking right side crankcase seal could cause that kind of damage. Maybe, because I left the oil in there so long, the oil that did leak by had a lot of those abrasive particles? I'm just trying to figure out why the bridge scored the piston. The plating is fine, however, it was somewhat more polished over the bridge and the polishing extended in a line the width of the bridge to the top of the cylinder. I'm going to be putting a new piston in here in the next week or so and I don't want to score the new piston as well. I have ordered a new left side seal and replace it as a cautionary measure, but I haven't seen the bike consuming much gear oil (it does suck some through, though). I ask for Mr. Gorr's expertise as he is the one who stated wear over bridges may be attributed to tranny oil blow-by. Oh yes, there were no holes drilled in the piston over the bridge, but the piston had at least 30 hours with no damage and then scored within 10 hours.

 

[snb73]

Some piston applications require a "cooling hole" drilled in the piston parallel to the exhaught bridge. This allows some of the fuel/oil mixture to cool the exhaust bridge.

I'm not sure if your application requires this. What is your fuel/oil ratio and current jetting?

 

[76GMC1500]

The bike has always been run at 32:1.

 

[CaptainObvious]

A scored piston is usually from dirt that got by the air filter.

 

[76GMC1500]

If any of you have the Gorr's Performance Handbook, in the 2-stroke engine rebuilding section it has a series of photos of piston failures. The one labeled bridge point looks like someone took a picture of my piston, except mine doesn't have the oil holes. It says it's caused either by a leaking right side crank seal or insufficient bridge clearance. Now I have some questions about bridge clearance. Does that cleance need to be .001-.003" larger than the piston clearance even though the piston may already be at .003" If it is .001.003" larger than the piston clearance, is it possible to lose bridge clearance as the cylinder wears and becomes more round? How is bridge clearance adjusted if it is insufficient? The piston showed no unusual wear for most of its life and then developed this wear mark within a very short time period.

I have a new right side seal coming, I will toss that in there with the piston just in case.

 

[RM_guy]

You need those 2 or 3 holes in the piston at the exhaust bridge. The Weisco instructions tell you exactly how to put them in. They don't have to be perfect, just make sure they are lined up with the bridge.

The holes let the fuel/oil mixture through to lubricate the piston at the bridge so if you don't have them it can seize and cause the scoring. They are strictly for lubrication and won't affect performance at all.

 

[griffbones]

For what it is worth, I really doubt the Amsoil caused the problem. I have run it in our two strokes for quite some time with no problems at all, and I mix at 40:1 which is less oil than what you are running.
Could be contamination from the tranny seal, as you stated this is something that Eric Gorr mentions in his book.

 

[snb73]

http://www.wiseco.com/PDFs/Manuals/MSeries.pdf

Lubrication removes friction, which cools (removes heat), from the exhaust bridge. This may solve your problem.

Kind of the way desert racers run a larger main jet too "cool" the piston at wide open throttle.

 

[76GMC1500]

For what it is worth, I really doubt the Amsoil caused the problem. I have run it in our two strokes for quite some time with no problems at all, and I mix at 40:1 which is less oil than what you are running.

Even if it were, I'd still run it. Amsoil is only $10 a quart and everything else is $16+ a quart.

 

[2strokerfun]

It is possible to lose the extra bridge clearance as the cylinder wears, but as you know, it is typically a very gradual process. And with the extra bridge clearance, you typically don't have to drill the piston -- the replacement pistons are usually drilled because most shops reboring cylinders don't relieve the bridge. Is there any evidence at all of any temporary overheating during your rides?? Is the underside of the piston crown darK? If not, I'm at a loss other than foreign matter from oil or air. Of course I've fixed many things and never for sure understood why they happened.

 

[whenfoxforks-ruled]

Come on, are you not some kinda engine mechanics student???? Bridge relief is critical,yes larger than the bore,this is a bad area for flashing build up,this area moves alot and is the source for cold seizure.Hey,have you ever thought about throwing a race on the big pirate barge? Good luck with your bike!

 

[76GMC1500]

Come on, are you not some kinda engine mechanics student???? Bridge relief is critical,yes larger than the bore,this is a bad area for flashing build up,this area moves alot and is the source for cold seizure.Hey,have you ever thought about throwing a race on the big pirate barge? Good luck with your bike!

Sure, I'm studying a lot of engine stuff for use on the barge and we do have 2-stroke port scavenged engines, but bridge clearance has never really come up. If you want to hear something interesting, about 2 generations ago of containerships used loop scavened diesel engines with all of the ports on 1 side of the engine. One problem with these loop scavenged engines was uneven liner wear, it would wear faster on the ported side of the cylinder because there was less bearing area. The solution? They run the ships with a permanent degree and a half list to transfer the weight of the piston and piston rod assembly from the ported side of the cylinder to the unported. The horizontal loads from the cranksahft and connecting rod are taken up in the cross head and not in the piston skirt like a normal gas engine, so there is no side loading in that sense. The piston and piston rod assemblies weigh upwards of 10 tons, so the weight is one of the most significant contributors to the side loading of the cylinder.

Personally, I think diesel sucks. I am a steam guy, but steam is going to be extinct here in a few years so I had better brush up on my internal combustion.

 

[whenfoxforks-ruled]

That is some kinda fancy engineering! My limited experience with carpentry and plumbing I would not give you 2 cents for all the engineer titles they pass out! They should come up with a class on how to use a little common sense! Now,what about a race on that pirate party barge,after you fix your bike! Do not stick a 00 ball hone in the cylinder until you get all the flashing off. Good oven cleaner works to some degree and so does muriatic acid,take care to keep it off aluminum and not for more than 20 minutes, use a q-tip if you use the acid. Your manual should tell you what the bridge relief for your bike should be,different pistons and cylinder materials have to be considered.Hell, I had a questionable jug and it goes to Eric,I have a 76 rm125 air cooler with problems,guess where its going,mo power!!!! If you put the ball hone in there it could remove the finish! Not recommended! The finish is almost indestructible,abused it can move the aluminum behind it, making an oval, or it can flake when its bad. Do you know about chamfering,also covered in most manuals.

 

[76GMC1500]

Everything is pretty well chamferred. There didn't seem to be any aluminum left behind, either. I will oven cleaner it like you suggest. I wasn't planning on honing. I threw new rings in there a month or so ago, despite the gouge because the budget didn't allow for a piston. Now, I have money for a piston so the bike is getting one (it still runs great, though). We're making a trip to Glamis for spring break and it is going to cost a lot in gas to get down there so I don't want to have anything break. I'll be doing about $250 in work for the bike which will include a new right side crank seal, chain sliders, piston, shock linkage bearings, and a lot of nuts and bolts type things. I'm debating throwing a chain on there.

 

[whenfoxforks-ruled]

HA,spitting a chain can be expensive,a good workout pushing your bike back though! From what I have seen on the coated liners is, if you can't see the crosshatch it could be covered by flashing. Since science has changed its theories on the very small and large,and some lucky stiff who made rhino bed coatings for hillbilly sports cars is coating government buildings,I am ready for my carbon cylinder and piston with some coating on it! Spring break,Glamis,I will find something to do,Jr should rap up a title for 250A class Motoplex INDOOR,or I could sneak off to Daytona!!!! 20 hours would make me cry Like a baby though,250 a gallon suks.com,I am not going to snivel over the 55 for 5 gallons of firepower for Jrs 450 though!

 

[76GMC1500]

I've got an almost new chain that will cost me a 12-pack ($6) of Keystone Light but it's only got 108 links. I'd have to find a few more links to make it useable. I'll probably just auction site one.

As for the cross hatching, there certainly isn't any visible but I think that's just because the plating is so worn.

 

[RM_guy]

You need those 2 or 3 holes in the piston at the exhaust bridge. The Weisco instructions tell you exactly how to put them in. They don't have to be perfect, just make sure they are lined up with the bridge.

The holes let the fuel/oil mixture through to lubricate the piston at the bridge so if you don't have them it can seize and cause the scoring. They are strictly for lubrication and won't affect performance at all.

I'll say it again. :bang:

BTW, Weisco doesn't drill the holes in all of their pistions because a lot of them are used in different applications. Some need the holes, other don't. That's why they have detailed directions on how to add them.

 

[whenfoxforks-ruled]

I'll say it again. :bang:

BTW, Wiseco doesn't drill the holes in all of their pistons because a lot of them are used in different applications. Some need the holes, other don't. That's why they have detailed directions on how to add them.

Yup,I have had to change my previous opinion of wiseco pistons,when they first came out I went through a few,oem was better.Now its turned,but vertex is still on my watch it list and the Y#$^%#@ I will find out this summer.I thought all 2 stroke pistons came with instructions,a caveman could do it!I fixed your spelling,your welcome.

 

[RM_guy]

Thanks for the spell check. I can't spell for crap. Spell check is my savior but sometimes it doesn't even know what I'm trying to say!!! :ohmy:

 

[whenfoxforks-ruled]

Tell me about it ,computers,I was to busy racing! The firefox browser is pretty slick,I was reading your post in a quote window and there is a red line,spellcheck,cool,learned that here also!!!

 

[2strokerfun]

Tell me about it ,computers,I was TOO busy racing! The firefox browser is pretty slick,I was reading your post in a quote window and there is a red line,spellcheck,cool,learned that here also!!!

Couldn't resist.