The Dodge Challenger Message Board

Hi ya mates
What piston clearance are you aiming for when final honing a cylinder?
I'm just doing a cheapo build on a 440 and I'm pretty sure max clearance is about 0.008". Blocks being bored 0.030" over. If using an aftermarket piston for street use do you follow manufacturers tolerances or is there a general rule of thunb for cast or foged pistons?
Cheers, Adrian

I would advise using the particular pistons manufacturer to establish bore to piston skirt clearance. The skirt, perpendicular to the piston pin centerline is where most pistons are measured from because a pistons skirt is actually barrel or oval shaped slightly, this reduces scuffing and helps to reduce piston rocking in the bore during running. Cast Pistons have a higher dome temp than forged pistons which absorb more heat internally, therefore forged pistons require more clearance unless it's cut for anti-expansion slots below the ring grooves ala Speed-Pro and the older TRW forged pistons. Today most modern forgings use a different approach and concentrate the strength in this area for forced induction of nitrous usage. 2618 Forged alloy requires the most clearance followed by the 4032 forgings. Cast pistons usually are fitted as close as .001 but forged slugs will need as much as .008 or higher, depending on the design, how it's ground around the skirts, if it even has skirts ect,, this is why I would contact the man. for specific clearances. My Keith Black 2618 pistons require .005-.006, like I said it can be more or less depending on many variables. Too loose and the rings will be overworked, to tight and you risk scuffing. The hone is vital to a proper surface finsih depending on what rings are used. Most are honed for a fine finish for moly rings, then this size is matched exactly to the pistons diameter, this then becomes 'married' to that particular bore. This is not always done but for a blueprinted job, it's a must.

Good answer Eddie!

I would have just said "depends on the piston".

Hi Eddie, that was a good answer. When you say (for exacting builds, such as blueprinting) that the bore is then sized properly for the piston going into it, how is this final sizing done? ie, is the material removed with honing? So if you have a piston that requires say 7 thou skirt clearance and the bore measures only 6 thou, is the final 1 thou removed with honing? If so, I assume you would have to be doing a torque-plate hone to get it this accurate?

Also,where do Hypereutectic pistons fit in? Say, the popular KB hyper-eutectics. Are their heat characteristics closer to a cast or a pure forged piston?

Thankyou.

Goldenblack440 wrote:Hi Eddie, that was a good answer. When you say (for exacting builds, such as blueprinting) that the bore is then sized properly for the piston going into it, how is this final sizing done? ie, is the material removed with honing? So if you have a piston that requires say 7 thou skirt clearance and the bore measures only 6 thou, is the final 1 thou removed with honing? If so, I assume you would have to be doing a torque-plate hone to get it this accurate?

Also,where do Hypereutectic pistons fit in? Say, the popular KB hyper-eutectics. Are their heat characteristics closer to a cast or a pure forged piston?

Thankyou.

First the piston to be used in a particular bore is measured around the skirt area or where the piston man. wants you to measure. Most are measured around the skirt perpendicular to the piston pin centerline. Then the bore is final honed to that size. Honing can be done by hand, but it's best to use a load measuring device so as NOT to burnish the the cylinder wall. You dont want to polish the wall, the wall must be finely cut with a directional finish with 400-600 grit honing stones, depending on the type of ring selected. Moly rings take a finer finish than plain cast iron rings, this allows for instant seating and the surface finish of the hone will hold oil on the wall, because of the directional finish. Usually this finish is 60 degrees cross cut. This is why the hone is moved rapidly up and down in the cylinder. This is to establish the surface cut of the stones against the cylinder wall. This aids in oil retention on the wall. Only about 1-2 thousands .001- .002 is removed during the honing process. Too light a touch and it burnishes the metal this is NOT desired. Too much force and you remove the metal that you need for the perfect bore. Typical piston for a 4.340 bore would actually measure 4.336 or something like that Adrian. The pistons bore clearance is built into the piston. However, as anything thats manufactured it's not exactly spot on which is why it should be honed or finish sized to a particular bore, not the other way around. Hope this helps and doesnt confuse. As for the Hyper-uetectics they can run a very tight clearance due to the silicone content in them. Silicone WONT weld, this is used to reduce the chances of scuffing. They are a great piston for street or strip and can run with as little as .001 bore clearance, however, the hyper pistons are STILL a CAST piston and DONT have the ductility as a forged piston. The ductility is desired in racing because if something happens like a dropped valve the piston wont shatter. It will instead deform and hopefully absorb the carnage and save the block. cast pistons shoudnt be used for high RPM's past 6000 RPM's or so and they must limit their nitrous or forced induction usage to very small limits. Forged pistons are used for their strength. The 'majik' question is how much strength do you need? Production pistons are all castings, except the 426 Hemi and Maximum Wedge engines. All else were simple cast slugs. If I were building a street only plant with performance in mind and 6500 RPM MAX limit, no NOx or blower, I would use the Hyper pistons for their great heat reflecting qualities. tight fit for reduced blowby. Of course to measure all this stuff will require a dial bore gauge, and a good quality micrometer with radiused anvils. The good ones are very expensive. A set of snap gauges wont work! Been there done that.

Goldenblack440 wrote:Also,where do Hypereutectic pistons fit in? Say, the popular KB hyper-eutectics. Are their heat characteristics closer to a cast or a pure forged piston?

Thankyou.

They are closer to cast i.e. tighter than forged.
But again, the piston manufacturer states what piston to bore clearance is needed.

More info can be found here www.kb-silvolite.com and the Book," Engine Blueprinting" by Rick Voegelin SA books avaliable at Amazon.com are very informative. (Please realize the book involves blueprinting procedures using GM engines, this doesnt really matter)..

That is very great and comprehensive info - I certainly hope Adrian reads it. Eddie, what do you think of Sealed Power rings - not sure if they even still make them. I think they were all file fit? Also, Perfect Circle rings - they don't use a normal gap?

Goldenblack440 wrote:That is very great and comprehensive info - I certainly hope Adrian reads it. Eddie, what do you think of Sealed Power rings - not sure if they even still make them. I think they were all file fit? Also, Perfect Circle rings - they don't use a normal gap?

Perfect Circle and Speed-Pro are both good rings. Ive used them both with great results. I personally like the Ductile Iron Moly faced or moly filled rings. The moly face have a Plasma-Moly coating placed on the top ring face. Plasma-Moly filled have a top ring thats actually 'filled' with the moly face coating on the leading edge. I dont know which is better to be honest. The last 3 engines I have built, a Ford 394 stroker and a Chevy 350 LT-1 both used Speed-Pro and Hastings Rings. I liked them both! My 500 Stroker used Total Seal ductile iron top ring plasma moly filled, standard barrel faced top ring, cast iron 2nd, stainless steel oil ring package. Keep in mind there are a myriad of choices in piston rings, backcut, beveled, barrel faced, chrome, ect,, it can get kinda complicated at times. I dont think you can get much better than any of the rings you or I mentioned, they are all good quality with race proven results. The most important thing is the bores are straight and true, the bore finish is correct for the coating or material you wish to use, the top ring gaps are set with enough clearance or gap to ensure they dont 'Butt' at higher operating temps. This is VITAL on hyperuetectics than any other piston type as they expand much more at the pistons crown or top of the ring groove more than any other piston design so the top ring gap must be increased a few thousands more than usual. The Chevy I re-built had hyper pistons with only .010 top ring gap clearance. I knew that because it was the only piston that wasnt damaged, they rest were damaged very badly, the block had to be bored .040 to clean it up,(they got lucky), it had less than 20 miles on it before it siezed-locked up. The 'gap' depends upon both the ring manufacturers and piston manufacturers directions. As in the hyperuetectic pistons which require a larger top ring gap. In this case, whatever ring you choose, you must follow the pistons direction first and foremost. "File fit' rings can gain power because each bore is gapped according to it's size. pre-gapped rings are a hit and miss deal, some bores may have a larger or smaller gap than others. it wont be a lot, but when you are building an engine for a very low leakdown rate, thus more cylinder pressure, the hand gapped rings should make more power. The factory of course didnt do this. There is power to be had from 'blueprinting' the pistons, bore, rings, rods crank ectt.

Can you possibly get a more comprehensive answer than that? I doubt it.
Eddie mate how do you fit all that knowledge into a head you once described to me as "kinda small and poanut shaped" ? Thanks again.
This site never fails to help !

Adrian Worman wrote:Can you possibly get a more comprehensive answer than that? I doubt it.
Eddie mate how do you fit all that knowledge into a head you once described to me as "kinda small and poanut shaped" ? Thanks again.
This site never fails to help !

Eddie
You are a Genius! When I read what you have to say, my head starts to spin.