turning the top of piston to lessen or remove the dish

[scootercat]

I have a question,and I asked the same in My build off thread....Has anyone cut the top of a predator212 piston and used a .020" longer arc rod to end up with 0 deck height?....I have the tools to measure everything,just have to go through the r&r of the old rod/piston deck height,then the same with the new rod/piston set up....I'm only asking if its been done or worth the effort,thanks....scootercat....

 

[Stangrcr1]

Before removing the stock rod/piston, measure how far down the piston is in the bore. Measure at the edge above the dish and measure again in the dish. Is the edge number even with the top of the bore or is it below deck?

Lets say your above dish to deck measurement is .003 below deck. Adding an .020" rod will push the piston above bore by .017". This means you can mill .017" off the ring around the dish for zero deck. If the dish is deeper than .017, which I think it is, you won't get flattop.

Regardless, it would be a worthwhile compression increase.

You can't just take the piston to a machinist and say "mill the piston flat". You and the machinist need to know just how far to mill it down.

 

[scootercat]

Thanks Don,I ripped them apart before I measured the deck height,so...I have to bung them back togther and check....I know the drill,just wondering if anyone had done this to the stock piston....I saw there's a rod and flat top piston arc sells,and You have to machine the piston to get the deck height needed but that rod is like .070" over and the piston is a weisco forging....I've already spent over six bills and still need carbs or carb,still undecided on two 22s or one big 36mm flatslide carb....I'm gonna work on it in the A.M.:laugh:....Scootercat....

 

[Neck]

You may be asking for trouble taking away from piston dome thickness. The more common approach is to deck the block and then deal with the change with custom pushrods. Do you know what the dome thickness is with your current piston?

 

[FirePowerMinis]

Yes, I've done exactly this mod. Dish was deeper than the .020 longer rod, but I wanted a .010 head gasket and .030 quench, so it worked out fine.

It's not rocket science. I guarantee that careful use of a belt sander would do the job.

http://sphotos-e.ak.fbcdn.net/hphotos-ak-ash3/556218_10151222404948177_1844990655_n.jpg

 

[youngengines]

This is what I do. Mock it up with the rod I'm using. Cut the deck down to the top of the piston, then cut the piston .015 to .020 for my deck clearance. There is not enough material on the Predator piston to cut it to a flat top (safely) in my opinion. I like a little dish for flame travel anyway!!! JMO

 

[roccosbike]

Thanks Don,I ripped them apart before I measured the deck height,so...I have to bung them back togther and check....I know the drill,just wondering if anyone had done this to the stock piston....I saw there's a rod and flat top piston arc sells,and You have to machine the piston to get the deck height needed but that rod is like .070" over and the piston is a weisco forging....I've already spent over six bills and still need carbs or carb,still undecided on two 22s or one big 36mm flatslide carb....I'm gonna work on it in the A.M.:laugh:....Scootercat....

The stock piston in my 8HP Tecumseh engine on my Bonanza was put in a horz vert mill and lightened to balance the assembly.I see no reason why you couldn't do what you plan on doing.When you make an assembly that spins with less vibration the RPM's are a lot easier for your engine to achieve.When I open up the Big Tecumseh engine that thing just Humms.

 

[Stangrcr1]

You may be asking for trouble taking away from piston dome thickness. The more common approach is to deck the block and then deal with the change with custom pushrods. Do you know what the dome thickness is with your current piston?

So long as he takes away just the area around the dish, there shouldn't be a problem. Making the dished area thinner, I can see that could be an issue.

I was just telling him how to do this and not have to deck the block, but still use the .020" over rod. Still get the higher compression and use standard pushrods.

 

[youngengines]

Just a FYI. The older Predator (plastic cam gear) has a thicker area between the top ring and the top of the piston. It can be cut all most flat. (Did it today)

 

[scootercat]

Yup!...Its .145" from the top edge of the ring land and the top of the piston....So if I take off the .050 dish and make it a flat top it'll still have .095" left above the ring....The thickness is .215" from inside to the bottom of the dish so its not taking away any strength from the top....Scootercat....

 

[Stangrcr1]

I wouldn't take the whole .050 off the piston. Measure how much the piston protrudes with the longer rod and cut that much off the piston. You will get higher compression than going to all flat top by retaining some of the dish.

 

[scootercat]

That's the plan Stang, I sure will! I have been doing some math and will have to wait till i get back to work to cut the tops on the pistons....Scootercat....

 

[PacMule]

Cut that piston.050,put 8 lateral or vertical gas ports to it,use the +.020 rod so you minimize how much you cut off the deck.

 

[scootercat]

Cut that piston.050,put 8 lateral or vertical gas ports to it,use the +.020 rod so you minimize how much you cut off the deck.

Pistons are cut....Thought about gas port holes....Using stock rings it might not help them much,just thinkin'....Scootercat....

 

[Stangrcr1]

Gas ports help regardless of which rings are used. Unless you somehow found TotalSeal rings to fit that piston.

Here is the question I have for you now. Since you have cut the pistons, are you going to deck the blocks? If you don't, I think you are going to end up with less compression than if you had just taken off the top of the dish to equal deck height with the longer rod. Maybe even less compression than stock...

Engine building is an art. You can't randomly machine parts, then throw them together, and expect it to work.

 

[scootercat]

Ahh,The art of engine building...Yes I'm going to deck the blocks because the deck is not square with the bore,and I'm the doing the gas ports too....and milling the heads to restore any loss of compression also....Scootercat....

 

[youngengines]

I just did a Predator a few days ago. Decked to the sleeve, used a older piston (thicker ring pack than newer) which has more meat between the top ring and top of the piston. Milled the piston close to flat (probably .010 dish left) Mod 2 cam, ported/milled head,26lb springs, aluminum retainers, bored the Predator carb venturi to .630,(left stock butterfly), aluminum flywheel, billet rod. Made 15.5hp, and I turned it 8100rpm on the dyno.

 

[scootercat]

Hey Youngengine,how much did Ya cut the heads?...What gasket are You using?....Going to mill My heads at work sometime this week and I'm decking the blocks also....Just juggling things around so I can slap 'em together and one more,where did You buy Your cam?....Thanks....Scootercat....