250 SL holed its piston. What to do?

[Darrell]

Thanks everyone, I'm humbled by all the deep thought you're investing in this issue...

Here's more of the backstory and latest observations:

Since rebuilding the Bottom End 3,200 miles ago I've been on the habit of riding the bike on highway stretches (up to 55 miles at a time) with no apparent stress to the machine.

Timing hasn't been touch since reassembly and the stator hasn't moved (unless it's a black box gone rogue defect). Can an ageing CDI develop a mind of its own and randomly change the timing?

Always ran with fresh Chevron Supreme 94 (Canadian), NO ethanol. Anecdotally, some car tuners are experiencing knock issues with Chevron 94 and switched to Shell 91.

Visually, LH crank seal is tight and dry with no apparent leaks or seepage. Seals were replaced 3,200 miles/2years ago along with a NOS crank.

Overall the bike's history of duty and maintenance is consistent, with the only obvious change, or variable, being the float failure.

I'm compiling a list of new parts for the rebuild -- it's already been stressed that I must eliminate whatever issue(s) caused this meltdown in the first place or I'll be walking down this road again.

P.S. At least one poster wondered how the cylinder looks; there's a skim coat of aluminum on the cylinder wall with some heavier metal transfer just above the top ring stroke limit.

[hodakamax]

To argue the technicalities of whether it was detonation or pre-igniton when the piston is melting is just silly. Sorry, both are manifications of the same problem, the cylinder temperatures have exceeded the limits of the aluminum piston and the lubricants. Also detonation would have probably been felt or heard. When liquid aluminum is circulating in the combustion chamber the plug is dead and all ignition is caused by heat whether post or pre. Victor, your arguments are correct that detonation and pre-igniton are two different things but in the real world they are all happening at once all with bad results I might add. Again my opinion! (and it gives us something to discuss!)

Maxie

PS--I purposely did not separate pre-ignition and detonation in my earlier post just to get things going. It's always good for a discussion. In my long racing career they were the same thing--Disaster.

[thrownchain]

The reason I asked about the cylinder is because if the bore is damaged you're have to factor in a bore and piston to the rebuild.

[ossa95d]

Everyone seems to agree that there was a cascade of events leading up to the catastrophic failure. The situation was most likely exacerbated by the 250 SL's inherent propensity for overheating, including both ignition and cylinder head design shortfalls. However the fact remains that Darrell put 3200 miles on the bike over the past two years under the same riding conditions without incident. This would indicate that there was some new condition that precipitated the failure. Everyone has contributed helpful information as to the cause but I agree with Max and Bruce that we were not there and only Darrell has the benefit of first hand experience. I also agree with Ed and believe Darrell's theory that the broken float may have become lodged under the float arm causing limited fuel delivery at sustained demand has merit and is plausible. That may have been the trigger. It doesn't mean it was the only factor though. Good job Darrell. My understanding is that you rebuilt the engine yourself before so you know what you are getting into. Good luck with the rebuild and keep us informed as to what else you find.

[viclioce]

Darrell:

Here's what Chevron has to say:


Other Abnormal Combustion Phenomena
Other abnormal combustion phenomena can occur in addition to knocking. Surface ignition involves the ignition of the air-fuel mixture by a hot spot rather than a spark. Potential ignition sources include glowing combustion chamber deposits, sharp edges or burrs in the combustion chamber, and an overheated spark plug electrode.

Heavy, prolonged spark knock can generate hot spots that produce surface ignition. A hot spot can ignite a portion of the air-fuel mixture before the mixture would normally start to burn. This may occur either before or after spark ignition. Surface ignition before a spark is called preignition. Surface ignition after a spark is called post-ignition. When preignition occurs, ignition timing is lost and upward movement of the piston is opposed by the high pressure gener- ated by early combustion. This results in engine roughness, power loss, and severe localized heating of the piston crown. If
it is prolonged, the localized heating can burn a hole in the piston.

Sound familiar?

Sounds like you aren't getting a true 94 Octane rating. I'm told, and research on the internet shows, that the Canadian octane rating system is neither regulated nor reasonably controlled. This makes the octane rating a crap shoot. If I were you, I would better your odds after the rebuild by adding an octane booster to your fuel.

Also the Internet says that Canadian "94" octane has up to 10% ethanol in it. So I would be extremely careful about choosing gas stations which say they offer ethanol free fuel. Ethanol makes your fuel burn hotter and gives you lower fuel economy. Though the latter may not be as important to you, you will always benefit from buying confirmed ethanol free fuel.

Good luck with your rebuild and keep us posted as to what you do and how it turns out. I would hate to see you hole another piston in 3,000 miles or so again. ; D Victor

[Dale]

Victor, Do you trust your local gas stations? Really? You are throwing a lot a stones within a glass house here in my opinion.
Dale

[racerclam]

The best prevention of this kind of problem is your ears detecting the sound of the little guy in your cylinder with the hammer , as soon as you hear it BACK OFF the throttle it only takes a few seconds to kill the piston. Even if you trust the gas you may have been running for years with no problem there is such a problem of a bad batch I have experienced this before with hot rods , you never know so just listen.

Rich

[olddogs]

I have had several SL250s and 250 EDs and chased the knocking in circles in all of them until I found a low tech solution. Octane booster, timing retarded, richer jetting, nothing helped until I added 2 extra head gaskets. Lower compression reduced the heat and the knocking. I rode the bike on the street for a number of years with this setup. Not rocket science, but it worked. As a street mount I never noticed any drop in performance.

[viclioce]

Now there's a handy solution! And based on your experience, I don't think any one would argue the point!

Dale, as far as questioning my local gas stations, I implicitly do! I make sure I get gas from top tier stations for my big bike and I also have hunted down a couple of stations which offer non-ethanol "enhanced" regular unleaded. We even had a case where a lady was fueling up a Saab regularly at a Pueblo gas station on the highway. She always bought their premium fuel as she was instructed to do so when she purchased the car. However, despite of her following directions she had engine failure twice and had to have the motors replace under warranty. The dealership inquired where she was buying her fuel. She stated at one of the Pueblo stations between Santa Fe & Albuquerque. A person was dispatched to obtain a sample of the premium fuel to test and the testing was done by an independent lab. The lab reported back that it was the most contaminated batch of premium fuel they had ever tested. Turns out, lots of folks stopped every day to by regular unleaded there, but so few people bought premium that the fuel sat in the storage tank, braking down, loosing it's octane rating and becoming contaminated with dirt and moisture in the below ground tank. So yeah, I'm pretty picky about where I buy fuel and no, I'm not very trusting of my local stations, as you ask. Even when I'm cruising the highway on a big bore 4 stroke, I look for top tier gas stations when out of my area. Knowing that smaller refineries may or may not put the quality control into what they produce is why. Nothing to do with being in a glass house and throwing stones. More, it's about being careful with the fuel you by, making sure it's not poorly stored, and listening to others who have had bad experiences with certain local distributors of fuel. I believe what I do based on observation and fact finding. And olddogs provides what appears to be a good resolution, based on his trial and experience solution. And that's probably the best opportunity for someone like Darrell to help himself avoid the problem going forward. Seems like a simple, but reliable approach to Darrell's issue by someone who did everything everyone here has discussed and came up with an alternative solution that has worked. ; D Victor

[Darrell]

I have had several SL250s and 250 EDs and chased the knocking in circles in all of them until I found a low tech solution. Octane booster, timing retarded, richer jetting, nothing helped until I added 2 extra head gaskets. Lower compression reduced the heat and the knocking. I rode the bike on the street for a number of years with this setup. Not rocket science, but it worked. As a street mount I never noticed any drop in performance.

Hmm, so happens that I didn't throw out the original gasket, plus I still have the new gasket from this last teardown. Do the stacked gaskets seal 100 percent, or is some seeping to be expected? Do you recommend a type of sealant between the stacked gaskets?

Somewhat off topic now, where I come from we used to think we could reuse (copper) head gaskets by heating them up, almost red hot, with a propane torch. I think the proper term is annealing. I don't know if this is an accepted practise nowadays.

[olddogs]

I remember trying 2 and not getting the knock completely out. 3 did the trick. I don't remember whether they were all new or not but I know its been 10 years and the bike is still being ridden by its new owner. The 250 gasket is a pretty stout gasket to begin with. The resulting lower compression will make gas quality and selection less vital.

[Bullfrog]

The method I was taught for annealing an aluminum head gasket goes like this:

- Use a short length of small steel or stainless steel wire through one of the bolt holes on the gasket to suspend the gasket. Clutch screw safety wire was the usual "go to" wire for this. (actually, suspending from two bolt holes helps keep the gasket from twisting in the wind while trying to do an even "coat" and heat job)
- Use a "richly" set oxy-acetylene torch to coat all of one side of the gasket with carbon black. I've used a kerosene lamp or a candle to do the "blacking" job.
- Then use the torch with a gentle (but properly adjusted) flame to heat the "bare" side of the gasket until the black evaporates away from the blackened side. I've used a standard home variety propane torch for this step. Be sure to keep the application of heat moving all over the gasket in order to NOT overheat any single spot on the gasket.
- Let the gasket air cool until ready for re-use.

Ed

[racerclam]

Adding torque grooves is proven to reduce the chance of detonation by improved combustion efficiency. At tdc the grooves help to send the air fuel mix that is( normally hanging out where it shouldn't be just waiting to detonate at the wrong time ) toward the center where it should be , then after the flame front begins the grooves also improve complete combustion by the turbulence they create. THis picture of the piston crown being clean all the way around the quench area where normally is only clean where the transfer port wash it is proof.

[squid on a 300]

if the cylinder base gaskets are fiber wouldn't it be easier to double them up or would that effect port timing because your raising the ports by raising the cylinder a bit.?

Bob

[Darrell]

Adding torque grooves is proven to reduce the chance of detonation by improved combustion efficiency. At tdc the grooves help to send the air fuel mix that is( normally hanging out where it shouldn't be just waiting to detonate at the wrong time ) toward the center where it should be , then after the flame front begins the grooves also improve complete combustion by the turbulence they create. THis picture of the piston crown being clean all the way around the quench area where normally is only clean where the transfer port wash it is proof.

Wow, I'm really grooving (lame pun) on this torque groove concept. Apparently this brilliantly simple idea was dreamed up in India only about 20 years ago.

Anyway my 250SL head has an offset combustion chamber, whereas the one Rich shows is conventionally centered. Does the torque groove concept apply anymore with an offset chamber, I wonder. Would just grooving the front third/120 degrees of the chamber/squish area assist the delivery of the fuel-air mix rearward to the offset combustion chamber?

[racerclam]

Groove it all the way around even in the narrow area, the concept works on anything, even on screwball Bultaco heads , they make your SL head look good.

Rich

[rtboone]

Greetings

In order to insure that my 250SL doesn't fail like Darrell's, I purchased new larger head gaskets ( Hodaka Dave ) and a new SC CDI per Rich's recommendation in an earlier post on this thread.

With the stock single gasket, I got 150 LBS compression and 130 LBS with two of the new. My plan is to ride with the two gaskets for a while and then go back to one to see if there is any difference. My thanks to Ed for the info on how to aneal the aluminum.

The SC CDI is a bit too large to fit into the stock CDI location, behind the coil , so I mounted it in front of the coil inside the frame gusset where the oil tank used to be. The wiring is long enough so that everything plugged together without any modification. If I was using the oil injection, I think that I could have mounted the CDI under the coil.

Hopefully Darrell's rebuild is going well.

Tom