Can I swap one set of .030 over pistons for another, without machine work?

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MileHighDart

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Ok, here's the deal.

I want to get a little more compression out of my teen, by swapping in a set of KB167's

My engine was built about 15 years ago, bored .030, and over the years it has set a lot without being run, been in two different cars, and probably only has about 6 or 7 thousand miles on it. I've pulled the heads, cylinders look good, no ridge at the top, and you can still see some of the crosshatch pattern on the cylinders from when it was built.

Can I just give it a quick hone, and drop in a set of the KB167 pistons, (after gapping the rings correctly of course)?
Or do I need to drag the block to the machine shop and have them measure everything to make sure the bores are the right size? Really trying to avoid any machine shop time. Except for having it balanced. I'm certain it is .030 over cause the old pistons are stamped on top.
 
Was the engine balanced before? Different pistons might throw that off. Also the KB167'S may require a specific piston to cylinder wall clearance. That may be different than what you have. Things to check.
 
I never had is balanced last time, but was planning on that for sure this go around. And on the KB's I don't know if they require the bore to be exactly ... size.
 
cast, hypereutectic and forged all use different piston to cylinder wall clearance. They also all have very different weights which will effect balance. Best bet would be to take bare block, old pistons w rods, crank, balancer, flex/fly and new pistons to competent machine shop and see what they say. Might only be a hundred or so to install new pistons with balance,assumingnthey are close to same weight.
 
That can only be answered correctly by measuring the pistons you want to use and compare to the bore size.
 
A cheap **** would at least weight the new pistons and compare them to the old to get that variable taken care of. Did it feel balanced when you last ran it? If so, you could get away with at least getting all the new pistons down to factory weight. take weight out under pin at meatiest part of piston, or get lighter pins. Not the pro way but we are not all made of money.
 
Most people dont have a cylinder bore gauge in their tool box since a decent one is over $400, thus the recommendation to take to machine shop. Most guys dont have ability to swap pistons unless you old ones are floating and the KB are floating as well. Its cheap insurance to spend a hundred or two with a real engine builder/machinist and make sure everything is 100%. Taking **** apart twice sucks!
If everything is good, it should be cheap and if its not, its cheaper than blowing **** up thats set up wrong.
 
To the OP. In theory yes you could just deglaze the block and install a different set of pistons. The standard recommended clearance is built in to the piston IE: 4.03 bore piston with recommended .008" clearance the piston is 4.022" now of course there are tolerances to take into consideration but for a inexpencive replacement engine like a crate motor they are not mesureing every piston and matching it to a particular bore that is only done on all out race engines. so depending on what you intend to use the motor for is what you have to decide. from the sounds of it your just looking for more compression for a fun motor. as long as the pistons are weight matched to each other as in most sets of KB pistons sold as a set are you could just hang them on the rods and gap the rings for the KB pistons I prefer SP hypers no special gap and run them.
 
Couldn't you reuse your rings? There already seated, just keep them with the cyl they came out of. Unless your gap is way off.
 
340doc said:
Couldn't you reuse your rings? There already seated, just keep them with the cyl they came out of. Unless your gap is way off.
Click to expand...
They won't be seated when you put them back because they won't be in the exact same position, but I've seen them reused.....
 
I have done it, I removed my High compression domed pistons for a flat top with valve reliefs. I did replace the rings, and made sure the ring gaps were correct. If you were boring the block 30 over, it is recommended to measure the pistons, and bore each cylinder to fit the piston with the proper piston to bore clearance. It depends on how "critical" you want to be and if you are into balancing and blueprinting. I am pretty sure that a matched set of pistons will be really close to being the same weight.
 
KB hypers want a tight piston to wall. Most any other stock type will want a wider piston to wall. Fatal issue? No. But it's not the best case and IMO you're better with what you have with low static than stick in pistons just for a point or two in compression. You don't say what year the 318 is,but if it's pressed pin you have to have them pressed off, the new ones put back in place, you have to balance it (you noted that already), and you have to give the new rings a surface to seat to. At minimum. So is that all worth 2pts of compression? You would be better served to take that cash and put it into either a better set of closed chamber heads that give you some compression, and better flow, or just port the heads you have.
 
mguner said:
They won't be seated when you put them back because they won't be in the exact same position, but I've seen them reused.....
Click to expand...
Hmmm, I was always under the impression the rings would sometimes move from the original position they were installed over the course of time. Gotta be honest, I never checked this.
 
You can do the piston swap, measure skirts of the out going, and the incoming to get an idea of where you piston to wall clearance may fall. This will only be a measure of gaining or loosing clearance; buy some long pattern feeler gauges, slide one in along side a piston (with out the rings) in the hole. Your piston to wall will be 1/2 the feeler gauge. Use a "Blueberry"/ "Dingle Ball" hone for new rings; IF you wanted to try reusing what you have, don't touch the walls. Just make sure you gap the top ring per KB specs.

Balance your assembly.
 
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