The toolbox

Piston Meltdown

My experience with cars has been that there's some warning before everything goes to hell in a handbag, but that wasn't the case with my Chang. It was perfect riding weather and I was on one of my favorite circuits, just putting along about 45 or 50 on a straight and level stretch of NY 53 when she let go. The sound is impossible to describe, but it was obvious this wasn't going to be a roadside fix. Luckily, there was a house nearby and even luckier, the owner was in his driveway. He allowed me to call AAA on his telephone and within an hour, the Chang and I were home, courtesy of a rollback from T&R Mobil.

As soon as we arrived I started removing the engine from the bike. The teardown immediately followed. The starboard side piston had broken up. The cylinder wall was gouged and the seat for the exhaust valve was chewed. The inside of the engine was full of debris.

So the jugs were sent to the machine shop for a .025 overbore and a valve job on the affected side. New pistons and rings were ordered. Meantime, the question that begs for an answer is "what happened?"

Oil (or lack of) isn't on the list. I check the dipstick before and after every single ride and change the oil way too frequently.

Running with too lean a mixture can cause overheating, and that's a possibilty although there was no indication of excessive heat. I balance the carburetors using ATST's procedure and have always had good results. The plugs looked exactly like they're supposed to as far as coloration and residue go.

My riding habits might be a factor. I don't baby this thing one bit. Most of the time I run full steam ahead, pedal to the metal. It's just a fluke that I wasn't on the day of the incident.

One possible culprit is the exhaust system. I had recently started running with a pair of 1" diameter straight pipes in place of the original fishtail mufflers. This modification sounded terrific and the bike's performance improved. It was noticeably quicker and more responsive without those baffles. What I didn't realize was the need for rejetting the carbs to compensate for a less restricted exhaust flow.

I'm not entirely convinced this is what did my CJ in but rather than take any chances, we'll be running with the original fishtails.

Some thoughts from fellow CJ rider Orvo Valila:


Welcome to the Club of Busted Pistons... ;-) It's a painful experience and a very Darwinian way of learning. In MC Long March there is probably not a single member that has not had a seizure of a sort on pistons and probably Yours Truly is in the top of our charts with five pistons altogether (though one was a broken valve guide that was leaning the mixture and not the fault of my riding as such.) However, I have to admit that your case seems by far the worst in piston destruction. Normally it is just the skirt and possibly a couple of rings.

For what it is worth and in case you're interested I'd like to add some previous experiences here. So here goes my Post Trauma Analysis:

Problem 1: Lack of lubrication. Oil probably is a factor here, not so much in quality and quantity but more not being where it's needed. Lubrication of CJ engines (as well as M-72 and the original R-71) is rather primative, and pressurized oil is led mainly to bearings only. There is a single oil passage to the left cylinder whereas the right cylinder is left pretty much to its own devices, relying only on splash lubrication coming from the crankshaft and big ends of connnection rods. So not all that much oil ever gets to the right jug.

Problem 2: Temperature of oil. Oil temperatures tend to rise after any longer period of riding 50 mph (80 km/h.) This seems to be a problem with M1 (Grey Slowspoke) and M1M engines. Part of it is simply an inadequate wet sump oil reservoir not giving the oil a chance to cool down. As you probably know already one of the most popular mod for CJs and Urals is a new deep sump with cooling fins. Real-life experience tells me that when oil temperature in the sump reaches 120 Celsius (approximately 250 Fahrenheit) you have pretty much 15 minutes of piston life left if you do not slow down. This is exactly the reason why I have an oil temp gauge installed in my M1 Super. I really wonder how the hell Afrika Korps ever did get by with their R-71s!

Problem 3: High rev count. Nowadays 50 mph sure does not sound a lot but back in the thirties maintaining that speed for any period was a small miracle due to road conditions. Therefore the transmission ratio is kinda on the short side to make sure you'd do all the uphills and tight turns nicely. Without any mods side-valve engines cannot really do more than 40 mph as a marching speed for any long period. Therefore as you surely have noticed, the next most popular mod is the long final gear and/or 4th gear. Drops the revs down nicely, though if you have both in an M1 you may run out of juice with a sidecar and a heavy load.

Problem 4: Cooling. In a side-valve engine the cylinder itself tends to heat up more than in an OHV. This comes from the fact that most of the cumbustion happens in the cylinder (and not in a chamber in the head) AND then the hot exhaust is lead out by an exhaust outlet which is in the cylinder and not in the head again. So the poor under-lubricated, overworked cylinder gets another punishment by being double-heated as well. And to make matters worse the clever Chinese engineers have designed the cylinder head gasket with something resembling asbestos inside. Perfect insulation to prevent the head from participating in the cooling process. A good mod would be to replace them with solid copper or aluminium ones. And the last straw being the right hand cylinder getting less airflow between the bike and the sidecar, probably not a big difference but if things are already on the edge, well...

And yes, if you have well-breathing exhausts and a leanish mixture, that does not help. There is a small adjustment in mixture built in the carbs by lifting the needles in the carb sleeves but not much. If you need to adjust the mixture any further, there is a chance to play with main jets (parts 32, 33 and 36 in the picture below)

As you probably have guessed by now I'm well in favour of mods and I already have the mentioned oil temp gauge and lengthened final drive in my CJ. I also have ordered high volume oil pump from ATST and a deep oil sump from Dong Tian Enterprises. Also, the 4th. gear mod is going into place this summer. I'm simply running out of pistons in my spare stash the way I ride. ;-)

Feel free to ignore what I've said above if there was nothing you already didn't know or if you feel that they don't apply in your case. If, however, you find any of it useful to yourself or someone else, all the better. CJ riders unite and all that. ;-)

And now it's Hey ho hey ho, off to work we (I) go...



Thoughts from Ray Costa regarding a problem that could lead to the same type of disaster:

Dan; I know you had a piston meltdown and I wanted to bring you up to date on current research.  You know that Ted Smith has an M1, and I just sold my trusty and somewhat modified M1-Super to buy an M1.  It arrived, and to my surprise I found I had one of the "chrome" engines, which is an M1 engine that has been polished and painted with silver paint, including the cylinder heads. The right side transmission plate is chromed, but badly. It arrived with the cylinders removed, and this was a very good thing.  I found that the pistons and bores had some fine vertical scratches that looked like the engine had been run with no oil.  I also found that the stock pistons had landings for five rings, but only had three rings installed.  I communicated with Ted, and Ted promptely tore down his engine and found that his bottom oil scraper rings had seized in the pistons and torn up the bores.  After two weeks of investigation, and a communication with Ed Korn, of Cycle Works, here's what we found.  (Ed knows more about old BMW engines than anyone except maybe Vech.  Ed rebuilt the crank on my R69S and bored the cylinders.  I bought my special BMW tools from him.) 

  • Ted's lower rings were jammed into the pistons.  The rings were wider than the landings. 
  • Ed suggests running only three rings IF you use a modern, 3-piece oil ring.  He says not to use four rings if both oil scrapers are of modern design.  He doesn't say what to do if both are of the old, one-piece type. 
  • Ed suggests fitting a top ring and leaving the second landing open.  My M1 came with the top landing open and the second ring fitted as the top ring.  The only effect of this is to slightly lower compression.
  • The stock pistons are not well balanced, and they should be balanced before they are used.
  • The left cylinder has three little holes on the top of the skirt.  This is for oil to drip in and keep the bottom of the skirt happy.  If these get plugged, bye-bye piston.  It is extremely important when fitting the left side cylinder that these be kept open.  The reason the left side has them and the right doesn't, is the pattern of oil flinging of the slingers.  On the right side it comes up from the bottom and gets everything nice and wet. On the left side it comes down from the top and starves the cylinder.  The three holes help. 
  • The new high compression pistons that have just become available have three rings and a shorter piston skirt.  They should cure all the ills of the ancient design.  Ted is fitting a pair to his bike.  I'm going to set my engine up properly from the get-go and than tear it down at the end of the season to inspect the stock pistons.
  • The M1 engines came from the factory with polished cylinder bores.  This is a terrible idea because the cross-hatching caused by a good hone job leaves crevices for trapping oil while the rings are breaking in.  This takes at least 500 miles, and more like a 1000.  With a polished bore you can get oil starvation and a seized piston.  The M1-Super and M1M engines I have put together came with properly honed bores. 

Additional information from Ray:

Here's some more information on CJ pistons.  While putting my M1 engine back together I found that the piston clearance in the cylinders was over .010" which is about twice what you would normally expect.  Indeed, when I overhauled my old Packard flathead straight eight it was about half that. I checked with Ted and he confirmed that his engine was very "sloppy" too. This was not the case with my M1-Super which had more normal clearances.  I think the Chinese are onto something here.  Remember that BMW stopped making flathead engines in about 1942 or 1943, and the R12s used in WWII and the M72s used by the Russians were very reliable in all kinds of weather.  I think the secret was the wide tolerance of the pistons.  The pistons expand a lot more than the bores when the engines run hot, and these flatheads run much hotter than the M1-Super OHV engine.  (I am also impressed with how much cooling there is on my BMW R69S compared to the M1-Super.  It has more than twice the fin area.) I think the secrets to keeping flathead CJ engines happy are:

  • Good, cool oil and lots of it,
  • Plenty of room for the pistons to expand, and...
  • ...don't beat the hell out of them.  There may be some piston slap until the engine heats up, so it's probably a good idea to warm these engines up gently before going for a long ride. This should also keep pistons skirts from blowing apart.

My M1 engine came with asbestos gaskets.  Don't see many of those anymore! I am updating the instructions to include my experiences with building up the M1 engine and the 6V electrical system.  I'll do a wiring diagram with colors as I put it together.  This is as much for me as everybody else.