KB107 (0.30) Pistons undersized?

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Trevor B

Trevor B

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I just got a call from my machinist who was cleaning up my 360 block for rebuild. He found that the brand new KB107 pistons I bought from Summit were actually 0.002" undersized, giving a clearance of about 0.004" in the hole. Apparently, according to a race builder he called to ask about it, some high(er) performance pistons are machined slightly smaller to allow for more heat expansion in racing?

Anyone ever heard of this/had problems with this? I saw nothing on Summit or Jegs to suggest a 4.028" bore for these pistons!

Also, is 0.004" piston clearance a deal-breaker? My machinist suggested that it could wear the rings out much faster in a daily driver...

Thank you for your thoughts.
 
The recommended clearance is .0015-.0020" This why you measure the block first to see what overbore you need, order the pistons, measure them and then bore the block. There can be a difference in the measuring tools used to manufacture the parts and the ones the machinist uses.

Summit is very easy to deal with. They will take them back and send you another set. Hopefully KB screwed up and not the machinist. Well the machinist already did screw up by not getting the pistons in his hands first.
 
The block wasn't actually bored, just honed. It was in good shape and already bored .030 over and his job was to clean/magnaflux and hone as needed, as well as balance rotating assy. I gave him the pistons/rods/etc. before he did the hone!
 
An already run bore that was cleaned up? Then it is simple, your machinist blew it.



My people are dying for lack of knowledge and leadership.
 
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the only problem I can see with the pistons is that they will make noise like a engine knock because the pistons don't use a offset wrist pin, its dead center ,but if you go with a piston with a offset wrist pin you wont have any problem, now they do recommend .004 clearance with power adders.
 
Hypers are designed to be tight. He screwed up by making the bore too large, or he screwed up by saying it was fine to hone and run. It will run, but I wouldn't want it that way. Maybe look into a forged piston or the Speed Pro hypers that are not as tight as KBs?
 
.004 is pretty much the limit of piston to wall clearance on 107's, not a huge problem, it will run there, might make a little noise due to piston slap. Being Hypers, they don't expand much when warm, which is why you can run them so tight.

You are doing a piston change.......just out of curiosity, what was the piston to wall clearance of the old pistons, what did you take out?

On off chance possibility, call UEM and see if you can get a set that measures 4.030? It's a possibility that you are a victim of manufacturing tolerances stacking up against you. A phone call doesn't hurt.
 
Are the pistons being measured at the gauge set point. There is a specific location on the piston where to do it. Many times it's .500" from the bottom of the skirt at a 90 degree angle to the pin hole. KB will specify the correct area to measure it. Other areas will measure smaller since pistons are tapered and can be slightly oval too.
 
Well, just called United Engine and Machine and they told me the correct place to measure the KB107 piston is 0.550" above the skirt. He also told me the "0.030 over piston" measures in at 4.0285"! So they aren't quite 0.030 over...
 
Trevor B said:
Well, just called United Engine and Machine and they told me the correct place to measure the KB107 piston is 0.550" above the skirt. He also told me the "0.030 over piston" measures in at 4.0285"! So they aren't quite 0.030 over...
Click to expand...

All pistons are undersize. The BORE is what to measure! A 4.030 piston will always measure small for the clearance. Wouldnt work too well if the pistons were the same size as the bore.
 
RustyRatRod said:
All pistons are undersize. The BORE is what to measure! A 4.030 piston will always measure small for the clearance. Wouldnt work too well if the pistons were the same size as the bore.
Click to expand...

Yes, I guess I hadn't thought of that. What to do now?
 
what do the bores measure? a used .030" honed block may be measuring 4.032". with an extra .002" the rings will not wear any big deal. I ran a 340 with used forged pistons that asked for .002-.004 clearance with .007 for years with no problem. run it
 
Trevor B said:
Yes, I guess I hadn't thought of that. What to do now?
Click to expand...
Simple, make your machinist do it right and take it out to .040" over. Exchange the pistons and rings for .040".
 
OR you could leave the block as is and exchange the pistons and pay the difference for forged.
 
RustyRatRod said:
OR you could leave the block as is and exchange the pistons and pay the difference for forged.
Click to expand...
It wont matter Rob. The forged piston will be smaller for the additional clearance, so the bore would still be too big. It would really be noisy with that much clearance.
 
for your inform
replicaracer43 said:
I'd find a machine shop that knew what they were doing
Click to expand...
I ran some kieth black hypers at .004 clearance once. they rattled a little till it got warmed up then quit and was quiet. you could always set it up to run a couple of degrees warmer too. that engine ran low 11.00`s too. 11 0hs to be exact
 
hmm..my last 340 was .020 over and the pistons were 4.058.
Shouldn't a .030 oversize 360 piston be 4.028?Block would be bored and finished honed to 4.030 or a hair less..and pistons should fit with .001 clearance?
..so your pistons are 4.026?
 
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Block was bored 0.030 over and supposedly ran well before I got it. It went to the machine shop for checking/cleaning/honing. Machinist didn't bore it. He told me it looked good and didn't need boring.

He did have it balanced: pistons, rings, rods (SCAT) and crank. If we have to go to 0.040 and return/replace the pistons and rings, it's gonna weigh more. I suppose it will need rebalancing again?
 
Machinist "looked" at it and said it didn't need boring? Come on man, this stuff isn't rocket science. It's measurements and math. Guy doesn't sound qualified.
 
Going with 0.040 over. Picking up pistons/rings, will return to Summit, will bring back 0.040 over pistons, machinist will hone to correct size.
 
If the piston sizes are what the manufacturer says they are, then the blame is entirely on the machinist for the excessive clearance, because he called it ok.

Pistons are machined to have the same weight across the available oversizes. Any weight differences from the oversize components should be well within tolerances.
 
Make sure the ring gaps are set to KB specs also, double check yourself if you have to, it is critical with those pistons.
 
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