Piston troubles

hydro X · 11-19-2009, 04:09 AM

Bill, I know its been some time since I`ve posted on this forum... but I need your help.
In my 787 race motor I have been running Pro X at 82.5mm. I keep breaking the piston around the ring grooves. My problem is being at .5mm over I can`t get rotax pistons, so I need an alternative to the Pro X I think. This leaves me with Vertex or Wiseco.( perhaps )

Am I missing something, like is there a cross-over piston from a sled or a better piston?

Mr. Bill · 11-23-2009, 10:18 PM

Well most guys use the Wiseco's in their racing 787 engines, but nothing will work if the damages are from interference with the rave valve or if it is detonating enough to break ring lands.

Do you have a good champher at the top of the ex port ? Good piston and rave clearances ? Piston to cly wall clearance ?Does the underside of the piston crown look very overheated and bluish ? Is your ex port too wide to support your rings ?
If everything is in good order, try backing some timing out of it at high rpm. Deto will break any piston if bad enough. I have seen deto break a crankshaft into two pieces.

I'd probably try the Wiseco's, but whatever piston you chosse, you cannot run it forever in a very highly tuned racing engine for too long before it will break. Not if you are really leaning on the motor.

hydro X · 11-24-2009, 05:00 AM

Bill, I didn`t think of of the deto. Tops of pistons and squish band is clean and no signs of deto, but it could be up through the rev range. I will take your advice and try a little less pitch and take some timing out of it.

Thanks for the help.

Water Worx · 11-24-2009, 11:13 AM

Go with the wisecos in the 800, the prox pistons are always breaking in the ring lands when used in a SS motor.

Bill O'Neal · 11-24-2009, 07:03 PM

I never have a problem with the ring lands on ProX pistons in an 800 motor. If you are having this problem it is because of some other factor like heat , clearances, deto from jacked up timing, overloading the motor, ect.

hydro X · 01-23-2010, 05:26 AM

Just an update Bill. I think I`ve found it, the rave bellow cap in the front cylinder was completey stripped of thread and so was the exhaust valve. I`m guessing this would have put alot of pressure in that cylinder as there are no contact marks.

Whats your thoughts .....

Mr. Bill · 01-25-2010, 07:10 PM

I am not going to bet the farm on that solution, although that is surely a problem needing a fix.
Was it just that one clyinder that caused you to break multiple pistons when used in that one clyinder ? Is there any signs of the other piston having excessive wear in or around the rings and ring lands at the ex side of the piston ?
Are your ex. ports modified ( ported ) in the clyinder, and if so, when ported, did they bring the exhaust port opening in the clyinder to more of a squared off opening and take out some of arch at the top of the port and has the port been widened ?

There is a fairly well known company, and many other copycat porters that uses that method when they port your ex. port in the clyinder for you. Miller has resleeved and re-ported many of those clyinders that came out of that shop and others for the very reason that you have described, after a very short period of racing or just hard use running, even in a recreational modified engine. We are seeing the exact same thing happen to their customers' pistons, and nothing will fix it except installing another sleeve, then reshaping the opening to give the piston, and especially the rings, the support they need when passing the ex. ports.

Improper ignition timing or an out of index crankshaft will allow unburnt raw fuels to be pushed into and ignited inside the exhaust port and it can overheat that part of the piston as it passes the port. The ring lands are relatively thin bands of aluminum after having the piston casting machined to allow the rings to have a place to sit into in the piston. That thin band of the piston, the ring land, is the most likely part of the piston to overheat and break off when exposed to a flame front coming back from the ex. port, when all flame should be confined to the head chamber and the piston crown and upper clyinder liner until that piston drops below the top of the exhaust port where it can escape into the exhaust port and manifold and beyond.

If it was that the ring lands where breaking at any portion of their 360 circle, and not just at the ex. port area only, you could surmize that the grade of the materials used in the piston casting where not up to the task, but that is not the case.
Another factor could be if the ring land grooves where to widely spread from top to bottom of each groove for the thickness of the ring that was installed into it, the rings could flutter in the grooves and beat the ring lands until they broke off, but when buying new pistons, the rings they send along with the piston should be the correct thickness to fit into the grooves, not too tightly, yet not overly loose with too much clearance between the ring and the ring lands.
Real professional race engine builders will always check each and every part for proper clearance when assembling a piston, or anything else, before using it in a high dollar racing motor. But rebuilders basically rely on the machine shop to make sure the pistons have enough clearances in the bores and most other critical factors involving a minimum and a maximum tolerance with only a few exceptions because some tolerances must be checked by the person building the motor. For instance, in an automobile type engine, the crankshaft and brearings must fit properly and not have too much or too less endplay. Things like that can only be checked during assembly.

After picking up your rebored clyinders and the pistons you left there to be measured for fit to allow the machinist to properly measure and bore and hone out the clyinder liners, ask him if he checked the ring end gaps too when installed into the clyinder liners. Most do not unless you specify that you want them to do it for you as a good machinist has good measuring devices and knows how to use them. He may even have a end gap grinder machine should they fit too tightly in the bores with less than the minimum gap clearances. Remember, everything in there swells up with heat, and you do not want a ring gap that is too little when it gets hot, or one that is overly wide and allows blow-by and a poor sealing job of the rings.

Then there is that old enemy of high compression vs fuel octane or quality and age of the fuel, along with ignition timing, especially retard at high rpm. In some of the pwc racing engines that are built here in Mel's shop, certain varibles require a completely different timing and jetting for each clyinder, and some require less backpressures from the tuned exhaust system. That is why you see guys changing the stinger outlet diameter to match the engine with the exhaust system. One size does not fit all motors, unless they are fairly tame and forgiving motors like bolting a good system onto an otherwise failry stock or only mildly modified engine.
Does anyone here remember the very first run of production FPP 720 exhaust systems ? How many rear pistons did you lose before you figured out that the exhaust stinger diameter was way undersized ? The second run of those pipes came with an enlarged stinger on the pipe chamber, and for the most part, that problem got solved.

Be sure to check the indexing of the crankshaft. I recently checked an oem 787 crank and it was 12 degrees out between the mag end and the pto end when more typically, I see 1 to 4 degrees to be about normal on an oem crank.

hydro X · 01-30-2010, 01:34 AM

Bit of read there Bill, heaps to consider.
The ring lands are breaking on the front piston only, sometimes all the way around but mainly to the sides over intakes. This happens to be the jug that has had the power valve problem too. I might have to add that egt`s are only mid 900`s at first look at WOT and only climb to 1020`s max. The stinger has been oversized and pipe shortened with exhaust tube up to 3 inch from there out to the back. On a good day I get 7700rpm or more, but have noticed I get trouble when my hot tach records a max of 8100 or more. I figure this is just a spike when racing in the rough stuff. Maybe the damage is done when the prop is out of the water?

I will take note of ring end gap and other tollerances that you suggested on the build and see how it goes. ( maybe set the rev limit lower too lol )

Thanks for your input, greatly appreciated.
I`ll give the pro x another go.

faster7581 · 01-30-2010, 10:02 AM

Quote:

Originally Posted by Water Worx

Go with the wisecos in the 800, the prox pistons are always breaking in the ring lands when used in a SS motor.

ive always had better luck with the pro-x, thats what i ran in my SS800 lopez 951 killer

hydro X · 02-09-2010, 06:10 AM

Ok.. checked the crank while still in the boat. I make it 5 degrees out at best. This would make the front piston more advanced than the PTO. The mag piston is the one that keeps breaking and runs a touch hotter,. My temps have never dropped down and give signs of deto. Going to pull the crank and get it set right and welded.