Hey all!

I recently bought a rebuilt LA360 short block built with .040 over silvolite pistons (dished). Since I want to up the compression and am considering the KB107 pistons. I know the KB's do not require as much cylinder wall clearance as the standard pistons might but coming to my question, can I purchase the same size KB piston, install/balance and run with it? Or do I need to run the next size up and size the bores to match the pistons? This is not a race engine and will just be fun street engine.

Thanks in advance

Unless you're blueprinting an engine build for optimal cylinder wall clearances, which you're not, than you can just use the .040" KB107 pistons. You do not want to go one size up as that will compromise your block and its ability to cool efficiently. As you mentioned, you should have your rotating assembly balanced for the new pistons set though. As with all Hyper Pistons, it's most important to run the proper ring gaps, so follow the KB instructions exactly for the gap factor.

Great! Thank you

From my experience with those pistons and all the trouble I’ve seen from them, you need to set the clearances up like they say. Too loose, they rattle around and break. To tight and they scuff and seize and put debris in the pan. I’ve seen the pin break out of the bottom and the piston explode. Set the rings up correctly or they will break the piston off right above the top ring. The valve notches are so deep, it makes a weak spot and it will break off. They don’t like rpm if anything is off. Too much timing and detonation and they break. They have common failures but they are people that love them. It’s almost like politics and religion when the debate starts. I’ve never installed them just repaired the damage or in some cases given the bad news and start with a new block. I guess people like them because they are cheap, idk. To me, pistons are the heart of the build. I use Race Techs and never have trouble.

Is an Icon piston an option and offer something that gives the CR you are looking for? You might be able to hone to the right clearance with a forged piston.

That would be my preference as well.

Mahle are another option...I put the 930271030 PISTONS, PowerPak Piston and Ring Kit (0.030" over, 1.0mm top, 1.00mm second, 2.00mm oil ring) in my 408 stroker build.

They call out 'bore x 0.0045' for "High Performance Street - NA" applications.

The piston material is their 4032 forged hyperutectic, so perhaps that's why their top ring instructions don't call out for such wider top-ring end-gaps???

So far so good in my ride...what really impressed me was the massive drop in turning torque between these and the Sealed Power HP116CP I used in my previous 360 motor with 5/16" top, 5/64 second and 3/16 oiling ring, here are the details:

TURNING TORQUE - MAINS : 8 - 10 in-lbs
TURNING TORQUE - 1 PISTON : 10 - 12 in-lbs
TURNING TORQUE - 8 PISTONS : 105 - 150 in-lbs

Not sure if they have the specific piston you need, but I'm thinking the sizing is pretty standard fare for SBM.

We have used KB pistons with good success even running 11 second ET's. In my opinion they are a very nice street piston. We've run piston to wall at .002". Follow the ring gap instructions. Many people get caught up in "the ring gap is to big I'm doomed". A few extra .00X ring gap won't bother anything, to tight is bad news. Now, what is your current bore size? It should be 4.040" if it's a .040" over bore. Wall clearance is built into the piston. Todays pistons are pretty accurate in their desired size. So by all rights if it was bored and honed properly it should be darn close. How would you know? Measure the bores.
Doug

All pistons have the clearance built in. so the bore is what it is. This can be pushed a little but it will never totally clean up unless bored. Now without a bore gauge and some numbers its a crap shoot. What are we talking about here a quickie with no regard to tolerances/balance?

If you have a piston clearance issue, I highly recommend having line2line coat the skirts. Actually even if you dont I still recommend it. had my old pistons that had excessive clearance coated last winter, and after seeing the results, I will have that done on every engine I build from here on out.

Interesting this comes up now as in considering those pistons for a 360 mag in a 3/4 ton truck with a 5speed behind it (granny low and 5th gear OD.... Something like an NV 4500.
I'm watching. Only difference is mine is 30 over not 40.

Welp I pulled the trigger on the Piston/rig set along with a set of Scat Rods, Speedmaster heads from Black Friday Sale and a Howards CL710581-10 230/236 with .531/.547 lift. It should be a fairly hot 360 for my 75 D100.

Make sure and have the balance check and corrected if needed