Does .001 really matter in a piston??

[zodiacblueS]

So I sat down to order the SpeedPro 2323 pistons for my 455 "S" rebuild and before I could pull the trigger, I took one more look at what is available for an unbored big block. As a rookie, and being unafraid to show my ignorance :confused:, I see Keith Black has a similar piston, with a d shaped dish reducing the head volume by 3 cc's. The price is within a few dollars of being the same. Will the KB 4.126 dia. pistons fit in the stock 4.125 cylinder? or would I need to have the block bored .001 inches??? What's the deal with .001 inches??:confused: The 70 to 90 dollar ea. pistons are out of my budget and needs, but if I can bump the compression a tenth or so, I'd like to do it now.

[mdkingsley]

I would not think that would matter there is that much wear in the block anyway. Have you had it to a machine shop for cleaning and bore check? They can tell you for sure.

[W451973]

No, that does'nt matter but what does matter is that your machinist mics each piston and measures each bore to make sure you have the proper clearance. Something you might not get form an out of round hi-milage bore. ;) Now the question does arise as to why you don't just bore the block oversize ?

[zodiacblueS]

The engine has a little less than 67 thousand miles on it. I knew the little old lady who drove it (my late mom) and kept it garaged for 38 years. Once I get it open I will evaulate the condition, but knowing what I do, I think things will look pretty good. Hopefully the short block can stay as is with the addition of higher compression pistons. Does anyone have good experience with KB pistons? I have read mixed reviews on the other forums.:eek:

[74hurstolds]

I'm willing to gamble that the short block will not be able to stay 'as is'. It's possible you'll have a ridge at the top of each cylinder. When you get it apart run your finger nail along the top of each cylinder and if it catches you need to get it bored otherwise when you fire up the new pistons, the rings will hit that ridge and break. If there is no ridge and you don't want to get the block bored over (this ended up being my scenerio) you'll want to invest in some measuring tools (regardless) and check for cylinder wear, taper, out of round as well as crankshaft taper etc. You'll also need to check the crankshaft and rod bearings to see the health of the motor. It's a big "it all depends". If it were me I'd get it boared .030 over and send the crank, rods, pistons to the engine builder to get cleaned up, pistons assembled and the rotating assembly balanced then assemble it yourself. I wouldn't feel comfortable throwing high compression pistons in a factory short block, chances are the money you threw at it will go down the drain because you'll have to do it all over again the right way.

I just did a 455 .030 over and each cylinder measured 4.156 +- .0005 all around. Each SP 2323 pistion I had measured 4.15375 (and was also on the box label). My clearances ended up being .0023 - .0028. The motor runs cool and starts easily, even when hot. I have J heads with a .040 head gasket and it runs on 93 just fine with a fairly large cam. It doesn't stink either. So the KB pistons may advertise as 4.126, but my guess is that they will end up being 4.12375 measured at the skirt. I used this formula to get these results. http://biosystems.okstate.edu/home/fharry/3211/900_Homework/CylinderTaper.htm

[zodiacblueS]

Excellent data and advice! Plus I get to buy some new measuring tools.:rolleyes: I love this stuff!

[74hurstolds]

Trust me it's worth the investment. I dropped $300 on them. Got a dial caliper, some micrometers and dial boar gauge. I started off just wanting to replace a rod bearing and ended up rebuilding the motor including a new crankshaft. It's a can of worms. All in all it cost me $1500 for a new crank, bearings, seals, pistons, rings, rods, tools and machine shop labor for polishing the crank, pressing the pistons on the pins and balancing the rotating assembly. It will be different for you depending on the condition of the crankshaft, rods and the block. Other things *will* come up. I saved money by assembling it myself (first time). I didn't have the block re-bored because there was no ridge and the specs were fine, so those tools came in handy for me. It was a huge learning experience for me so I'm willing to share my new found knowledge gained from the past 6 months of reading forums and getting books etc. The motor runs like a raped ape! Just was ripping around in the car. :D

[ragz442]

Excellent advice. I've thrown earlier heads, intake, headers etc. on a stock bottom end before [drive one now] on a budget. But if your going to install new pistons and you have it down that far, you might as well do it right.

[Cutlassefi]

[QUOTE=74hurstolds;41246]I'm willing to gamble that the short block will not be able to stay 'as is'. It's possible you'll have a ridge at the top of each cylinder. When you get it apart run your finger nail along the top of each cylinder and if it catches you need to get it bored otherwise when you fire up the new pistons, the rings will hit that ridge and break. If there is no ridge and you don't want to get the block bored over (this ended up being my scenerio) you'll want to invest in some measuring tools (regardless) and check for cylinder wear, taper, out of round as well as crankshaft taper etc. You'll also need to check the crankshaft and rod bearings to see the health of the motor. It's a big "it all depends". If it were me I'd get it boared .030 over and send the crank, rods, pistons to the engine builder to get cleaned up, pistons assembled and the rotating assembly balanced then assemble it yourself. I wouldn't feel comfortable throwing high compression pistons in a factory short block, chances are the money you threw at it will go down the drain because you'll have to do it all over again the right way.

I just did a 455 .030 over and each cylinder measured 4.156 +- .0005 all around. Each SP 2323 pistion I had measured 4.15375 (and was also on the box label). My clearances ended up being .0023 - .0028. The motor runs cool and starts easily, even when hot. I have J heads with a .040 head gasket and it runs on 93 just fine with a fairly large cam. It doesn't stink either. So the KB pistons may advertise as 4.126, but my guess is that they will end up being 4.12375 measured at the skirt. I used this formula to get these results. http://biosystems.okstate.edu/home/fharry/3211/900_Homework/CylinderTaper.htm[/QUOTE)


First of all the rings will only hit the ridge if the rings are higher up in the new piston. What happens in a worn/tapered bore is the rings wear out faster because they have to move in the piston more in order to stay in contact with the cyl walls. Combustion/compression pressure keeps the rings against the walls.

Please don't generalize on piston size/clearence. You're talking about 2 totally different piston configurations, are you familiar with both? I am.
The SP is made from an older alloy. The KB is made from 2618, a much lighter stronger, and more durable alloy. Very good pistons in my opinion, I've used them 3 times in the last year, no issues.
The SP is also heavier, much heavier, with a different skirt design, that contributes to piston thrusting that will be a bit different than the KB. That will also change the clearence requirements as well. Plus the KB uses 1/16 rings, the SP 5/64. Use the KB.

W451973 is correct, have your machinist check everything then go from there. That's always the best way. Balance it and do what's necessary and you'll be able to get by with only having to do it once. If you really want to do it right it should be done with a BHJ Bor-Tru plate and honed with a torque plate like I do it.
Like the saying goes, "if you didn't have the money to do it right the first time where are you going to find the money to do it right the second time".

Jmo.