340 build questions

It’s been awhile since I’ve built a 340 and with all the non stock options out there, I’m looking for options. What I’ve got is a messed up budget build from a customer. He has a stock Scat 360 (3.58) stroke with the 340 mains and the good old KB 107 pistons that started coming apart. These builders that sells these cheap pieces of crap are my best customer. Why someone builds a motor and uses pistons that cost $250 is beyond me. These started to came apart and he caught it before it exploded. Right now we’ve got a stock set of 915 heads that need lots of works and I’m thinking aluminum. I noticed Edelbrock is out of stock. With the 7cc Grand Canyon valve reliefs in the KB pistons, I’ve got 9.16 CR with the Stock 915s at 72 cc. I’d like to use stock rockers if I can. We’ve got some parts that can be saved. The cam is a 222 @.050 duration Comp. the intake is an RPM airgap and Holley 650. Any other head choices out there with stock rockers as an option? I’d go for ductile iron if they still make them.

….. 107s are prob what’s in our iron headed 10sec mill.

Interesting post though and I’m wondering myself if there are even “middle shelf” parts available anymore? The above mentioned mill had 273 rockers on it for years, but we “upgraded” to some crane non roller adjustable rockers sometime along the way.

What was the top ring gap set at? The only failure I have seen with kb hyper is when the wrong gap was used, and I've seen it more than once.I have used them in engines that are still going strong after 25+years of hard running. Depending on the performance goals are there is nothing wrong with whats there, just needs fixed and done right.

107 is an awesome piston for the money. Some people cant make ANYTHING work though

I have KB's in my old motor. The thing I found is you need to widen the gap until you think it's way too much, then file a little more. People told me you can't spray them, but they are wrong.

Keeping things irrelevant and off topic…. In my summertime boredom I’ve come to the conclusion that if the op wants a “simple” aluminum head upgrade it’s going to involve a new set of aluminum roller rocker arms. I know that he mentioned using the “stock” rockers but I can’t see anyone going to an aluminum head and use stamped steel rockers (am I wrong?). I’ll be honest… it’s been years since I’ve “shopped” for a “warm” combo, but it sure seems the days of ductile adjustable rockers is dead.

Roller rockers add cost and they seem to be more noisy. My TA ductile arms work fine. The valve reliefs are cut so deep on the pistons, I don’t see how they lasted the 1000 miles they did.

Well as far as “stock” rockers go those are the ones a guy would want. Have you checked into the Trick Flow head? ….they’re probably “not in stock” too. Seems to be the pattern with anything these days. Maybe the hoarder Pittsburghracer will part with a set of the speedmaster heads he has .

As to the KB107 pistons, what was the piston-wall clearance? They call for very tight clearance. Did the skirts break?

I have not gotten this motor apart so I can’t tell you the clearances yet. And yes I’ve heard about how careful you have to be with clearances on these pistons. End gaps, skirt clearance, etc. I don’t use cheap pistons. My shop won’t either. I’ve know half dozen of my friends who have tried them and they have failed in some for or another. And you are correct, some shops can’t get anything to work right. This is the fifth motor in a row that I have built that some other shop has done something wrong and the engine has failed. And they are coming from all over. I got a 68 440 in a trade deal the other day. It’s still together. I pulled the valve covers and noticed that the oiling holes on the rockers was install UP! What a shame. My shop was started by an old Super Stock racer who built race motors to keep his race cars going and ended up a really good business. To answer your question, though. The piston broke where the valve relief is cut at the rings. It is cut way to deep, in my opinion, for a hyperutectic piston. They are not meant to be as strong as forged. One good thing. It broke apart in a million tiny pieces instead of staying one big piece and punching a hole in the head. I’ll post a picture from my phone.

And before someone says detonation problems, none of the other pistons showed any signs of that. I thought that at first until I realized that the debris had bounced around through the intake plenum and blasted the pistons that on the same plane of the manifold.

Yep, not enough ring gap is common cause for that type of failure. It's not cheap pistons, it's poor attention to detail and following instruction. The old .004 per inch of bore doesn't work on KB hypers.

Used a bunch of those same pistons and juiced that bejesus out of them with no issues. It's not the piston IMHO.

Again, we’re just guess with our prof. I don’t believe in cheap parts. If I remember correctly, when I bought my headlight relay kit from you, you said you chose to buy better relays because they did not give you trouble. That I respect.

Honestly, I don’t know why I even mention these pistons anymore. I get sucked into this debate every time I find one that has failed. It’s almost as bad as the covid vaccine debate. 🙄. Just my humble opinion. I called a major engine builder the other day that is world famous and he agrees with me. He won’t use them either. I’m in good company, I think. Maybe I need to mention politic and religion so I can get this one locked up. Lol.

for those who are still following and willing to learn that is from a butted top ring (not enough end gap) Granted a forged piston would not have done that but neither will the kb, at the correct gap.

Yes and the reason is failures on the cheap relays.

Never had the issues you've seen on my stuff using those pistons. Don't know the answer for your experience. That piston damage is a classic not enough ring gap issue. Std piston gap 4-4.060 bore .016-.018, KB's need .026-.030 for the same bore.

As I say all the time, "Pick your parts, Pay your money." Use what works for you.

All good here, hope things go well with what you are working on.

I believe the top ring of the KB Hypereutectic piston is placed higher up toward the piston top (Closer to combustion heat) than traditional OEM and other cast replacement pitons. I remember reading how many would use the ring end gap spec for a factory cast piston or aftermarket cast piston on the KBs resulting in not enough gap and thus ring ends butting.

I thought KB added the extra material between the top and second ring for some reason I don't remember now and it was the second ring needing the extra end gap, not the top ring

Well I’ll get it apart and do some measuring and let you guys know.

No, extra gap in the top for the exact reason you see in the picture.

Are the current pistons below deck and by how much?

I agree on the valve reliefs I just checked a extra KB hyper vs the icon forged for the same basic application and the hyper is about .040 deeper. And with this being a small bore piston im getting a rough approx .040-050 thickness to the land. However I use these KB399 domed in my bracket car with a 150 shot ramped and a top gap of .038 with no issues.

Now the Icon Forged with the same 3.950 bore I spray 250 on ramped with a top gap of .028.

Never an issue with either.

I think you have a lot of options going forward, Id almost look for a proper oem 340 rotating assembly and sell the crank to someone else trying the old school mini stroker trick.

I snapped a few pics of the hyper vs the IC Forged and yes the hyper with the oem ring setup is just middle road over a stock cast but they dont make any high compression 318 cast pistons although even stock cast pistons can last and take some abuse.

This whole ring gap is nuts on the hyper and most just cant force themselves to put a mild motor together with big or bigger then poweradder gaps.

What I find interesting is there is some great lit. out about how big a ring gap can be and still work.

A first hand example I can share is in my youth I took a sd455 and re rung it with a simple hone job, it had .0055 bore taper. I used oem shelf moly rings and it never smoked or burned a drop, think of the gap change with over .005 taper so what the gap would go more then double oem?

None the less the lit, showed a minimal power loss with the bigger gaps, they said 1% over a filed set. This article was in engine builder mag and had info from oem ring makers on what some of the high limits were for some older stuff we use.

I snapped a few pics of the hulking hyper vs the ICF although the Icon is a stroker piston but both a 318-040 bore.

Next to the Forged... why cut the hypers so deep and not the forged? These are both zero deck with a blueprint cut deck height. And they wonder why the hyper will hold so much heat.

Sorry Cab, if you build a motor with hyper's and leave standard end gaps you will break some stuff. The hyper pistons retain more heat in the top. Therefore you need more end gaps. Why would you put more gap on the second ring? Just follow the directions that come with the pistons and you will be good.

I put a cheap set of hypers in a 390 ford for a friend in 2012.They are still in there in one piece. Instructions said to gap at usual spec. I opened them up by quite a bit instead. I'm the only one who knows they are wider, if they were too tight and lifted the land, everyone would know.

I've learn that wider end gaps are way better than a tiny bit, even .000010, to tight will kill parts
I am not a fan of cast hyper type pistons either
Forged pistons live longer with the same abuse
I do know that there are no parts made that can't be hurt by the right, parts killer customers, persons
I thought I remember reading the instructions on a set of the early KB hyper pistons saying to open the 2nd ring gap way up, not the top ring