Good piston design

Curious to find out if we have a better option on pistons as we are using CP at the moment on 1471 @ 31%, 28 lbs, 8000rpm max. 10.6:1 comp,BBC 4.50 x 4.00 stroke in a circuit boat which Wizbang would be very familiar with. The CP's didnt last long before they started developing cracks around the edges next to the pin bosses. They also started cracking on the inside of crown around the pin boss top were it meets the crown which eventually separated the crown from the rest of the piston and OUCH!!. Its not a drag race engine as it runs for around 3-4 minutes per qualifier or race so they are subjected to a bit more. Thanks for any info.

 

I had a set of pistons crack the same way, I admit, I was beating on them too hard for what they were, I went to Arias, they built me a set that are working good so far, they were a little expencive.

I think I was overspeeding the piston not overpowering it.

 

Most everyone I know run J E pistons, we use them in our grand prix boat motor, and the K boat. S/S, Pro/S, no problems

 

We are a fan of Ross Pistons. Over the years we have seen the same issue as you speak of. I believe it to be a fillet and or forging design issue.
Darren

 

I'll vouch for Ross, especially the coated ones, tried them ALL!

 

x3 for Ross

 

Thanks guys for the input. Spoke to the guys at CP & they said that the cut they make along the side of the pin can be left thicker which would mean a generous radius could be made there but I don't think that will solve all the issues. Does anyone have a good contact name @ Ross that knows what they are talking about, yet I have heard that Arias are suppose to make decent blower stuff too. Been around for some time now.

 

All my ross's crack like that around the pin boss. Figred it was detonation probably?

 

We ran C+A pistons in TAFC, 46 llbs boost, 10,000 rpm, cracked the same way. I was told it happens because during the exhaust stroke, you are throwing this [HEAVY] piston up the bore without any real resistance. Then you stop it dead and pull it down the bore, which trys to rip the top off the piston off of the wrist pin. Thats how the cracks form. Don`t know how long they will last that way, We threw them in the garbage after 25 runs.

 

Cracks around pin boss

There hasnt been any discussion on this thread about wrist pins . Is it possible a flexing wrist pin is working the piston in the boss area leading to cracking?

 

Very good observation Jim, but the pins in use are 9310 with .220 wall

 

I've always been told that bin poss cracking was from the fact that the wrist pin was flexing too much, which caused the piston to flex and eventually crack.

Remember you make much biggers gains in rigidity going larger in OD vs a small OD and thicker wall. Also, as far as how much a certain steel flexes, steel is steel is steel. High strength steel flexes the same as normal steel, for instance a 100,000 psi tensile steel flexes the same as a 36,000 psi steel even though it is 2.8 times stronger.

The other thing is you could make the rods pin end wider to help minimize the pin flex.

 

I agree 100% on the larger OD against small OD with thick wall.
The steel is steel is steel thing I strongly disagree. A high grade alloy steel with the proper heat treat is way less prone to flexing. Granted all steel will flex but the amount is what counts. There is a point in heat treating that steel can become brittle and break easy in a flexing enviroment. Try to bend or flex a high speed cuting tool like a good 1/2 drill it will just snap off. Next try the same on a good quality 1/2 pry bar it will flex a fair amount not break and return to normal. Now take a low grade steel like A36 rated around the 36000 yeild the same 1/2 inch it will bend or flex easy and stay where you bent it.
Or maybe I am reading the statement wrong.

 

Yea i giggled a little too about the steel is steel is steel thing
I work for a steel mill, many different alloys make different types of steel
so your plain ole a36 steel compared to HSLA is light years apart

 

Ye ye, giggle all you want...

I was not talking about strength, as tensile numbers do not describe how it will flex. The difference between the best alloy and the worst alloy in regards to it Modulus number (the number that explains flex) is about 10%. I hadn't thought of heat treating and am not sure how that effects the Modulus number.

Look up the tensile numbers of 6061 alum and of 1018 steel and think about which one flexes more vs which one has the higher strength number...

Hindsight I should have left this alone and let the people that make the pins explain it.

Meanwhile I will stick to my 1.094 x .250 wall pins and lightweight Arias pistons, I have been really happy with them.