Machining Piston to Avoid Seizures

Tech Carlo Catalanotto


Does the top diameter of my piston need to be machined in my engine?

Well I have been asked this question so many times that I figured it was worth some time answering it with a write up. The simple answer is yes, the explanation why is not quite as simple, but I will try to explain the best that I can from some of the knowledge that I have learned over the years.

This simple modification is done on a lathe by removing .002 to .003 from the outside diameter of the piston starting at the

Cutting Piston on Lathe
crown of the piston to .250 below the ring land. It does not increase horsepower it is just a simple fix to stop our engines from having the piston seizing in the bore and destroying the piston, ring, cylinder and sometimes more.  It is commonly believed that the reason we have seizure problems in our engines is because our cylinders were designed as air cooled units and that the conversion to water cooled cylinders is what has caused the problems. The theory is that the piston now expands at a faster rate than the cylinder because of the addition of water cooling around the cylinder keeping it cooler than it was originally designed for and without the heat it is not expanding at a fast enough rate to keep adequate clearance between it and the piston. This is not a bad theory, but not correct. With a lot of recent exposure to the RC style air cooled engines, I have learned that the air cooled style engines have more piston seizing problems in the bore than any of our marine water cooled engines so that kind of blows that theory out of the water so to speak, no pun intended.

The true culprit of our problem is heat expansion on the crown area of our piston. With the addition of tuned pipes and the recent wave of modifications to the piston to make it lighter has only made the problem worse. The crown of the piston with heat expansion will grow more than the rest of the piston even in stock form and there is no simple way to cure this problem. When engine tuners remove material from the inside of the piston area to make the piston lighter it actually makes the problem worse as you have less aluminum to absorb and disperse the heat from the combustion cycle. Some would believe that if it

Stock and Modified Piston
were possible to make our bore a little larger or a smaller diameter piston then the problem would be solved. While making the bore a little larger is not possible in our application neither of those solutions would work correctly. If the bore was adjusted or we used a smaller diameter piston to cure the problem then there would be too much clearance on the piston skirt that would cause other major problems such as the piston rocking in the bore, the ring not sealing because of the rocking and the piston allowing seepage through the ports that it is supposed to be sealing shut. The more research I did on this subject the more amazed I was to find out how long this modification has been used over the years in all sorts of racing 2-stroke engines. From the information I have gathered it has been done to high output 2-stroke racing engines for at least 35 years. While I do not believe this is the most optimal way for a piston to be used it is the most practical and economical way that I have found so far for our situation. Without the manufacturer producing what would be quite an expensive tapered piston for our relatively cheap engines, then the only way we have found so far to cure this problem is by machining the top diameter of the piston. I hope I have helped shed some light on this procedure and helped to save some of you from unnecessary problems in the future.

CC Racing Engines


Editor Note:  Here is a short article on how I made a fixture for the piston to turn it in a lathe. M.L.  CLICK HERE to go to article .
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lohring's Avatar
lohring replied the topic: #2936 9 years 11 months ago
Maybe the dyno runs broke in the piston before the racing heats. The runs are very short and nothing has time to get hot. I advocated for G1 engines for the class in the Propwash. This would eliminate a lot of the fights and stupid disqualifications I see coming. Years of running 'stock' outboards gives me a dislike of stock engine rules. In 2002 the NAMBA Nationals B Stock Tunnel winner was disqualified for a part substitution. Don't change your Zenoah bolts or water fittings.

Lohring Miller
CC Racing Engines's Avatar
CC Racing Engines replied the topic: #2933 9 years 11 months ago
lohring wrote:

Machining ANYTHING on the engine would make it not stock and would violate the thunderboat rules as written. My thunderboat engine has 50 dyno runs and 4 heats of racing with no sign of piston problems.

Lohring Miller

Consider yourself one of the lucky ones as I have repaired more engines then I can count from this problem. Some of them never have a problem and others happen right away. It is just luck of the draw. In my opinion those rules should be adjusted to allow that modification only.
lohring's Avatar
lohring replied the topic: #2932 9 years 11 months ago
Machining ANYTHING on the engine would make it not stock and would violate the thunderboat rules as written. My thunderboat engine has 50 dyno runs and 4 heats of racing with no sign of piston problems.

Lohring Miller
Steve Seebold's Avatar
Steve Seebold replied the topic: #2931 9 years 11 months ago
Does that mean it would be legal to turn the piston in the Classic Thunderboat class?
Scott's Avatar
Scott replied the topic: #2930 9 years 11 months ago
We are on the same page :P .... As you have noted the area that expands most is indeed the piston dome. The mod we do under cutting the diameter around ring is our ONLY solution to an engineering issue NOT addressed by the manufacturers.
In High Performance engines built for RACING duty, pistons are machined either tapered dome to skirt, And / Or cam ground that makes them No Longer Round. Cam grinding is done for the SAME REASON as our diameter turning. The material is thicker around a pistons pin boss area and therefor expands more than the thinner thrust surfaces 90* to either side of them.

BTW, Great seeing a legitimate engine tuner talking about things that get over looked or ignored due too ignorance of such things.

:woohoo: Regards, Scott

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