Diagnosing the Hemi Piston to wall clearence

[Paul_G]

I have the Hemi dissasembled. Trying to diagnose the failure. I bought it in running in a car this way. I have no history of what happened to it. It was running with three down holes, and it had an F1 procharger on it.

Getting it apart we found three broke piston top rings and lands. Sent the block to my local machine shop. They measured the bores to be from 4.310 to 4.312. I measured a piston to be 4.3" using my caliper. Not exact but close. Piston is probably a 4.310 which is a standard size for my rod and stroke combo making it 526ci.

The engine is using a 7.1" rod, 4.5" stroke crank. With 4.310" piston it works out to be a 526ci engine. This brings me to think about piston to wall clearence. Should have about .005", is that correct? With a piston measuring 4.3", and the bores at 4.310 to 4.312. This engine had no chance of survival. Or my measurement could be off. I dont know who built it.

 

[Bird 426]

I don't have your answer. Wondering why the other 5 are not broken. Sounds to me like the ring gap wasn't wide enough. What are the ring gaps on the 5 that lived?
I had a Roots system on a boat engine, if you push it, you'll break it. Maybe it got lean. What pistons lived, front or back of the engine? Or, maybe FOD got into the engine somehow. (debris) It's a learning curve.

 

[pnora]

I have the Hemi dissasembled. Trying to diagnose the failure. I bought it in running in a car this way. I have no history of what happened to it. It was running with three down holes, and it had an F1 procharger on it.

Getting it apart we found three broke piston top rings and lands. Sent the block to my local machine shop. They measured the bores to be from 4.310 to 4.312. I measured a piston to be 4.3" using my caliper. Not exact but close. Piston is probably a 4.310 which is a standard size for my rod and stroke combo making it 526ci.

The engine is using a 7.1" rod, 4.5" stroke crank. With 4.310" piston it works out to be a 526ci engine. This brings me to think about piston to wall clearence. Should have about .005", is that correct? With a piston measuring 4.3", and the bores at 4.310 to 4.312. This engine had no chance of survival. Or my measurement could be off. I dont know who built it.

Pistons should be measured and fit individually to each hole. There would be specs for clearance based on the application. The pistons also should be built for the application. Type of casting ring heights and sizes are important. Lots of people slap an engine together ignorant of what it should be and wonder about the failures.

 

[dodgeboy440]

I have the Hemi dissasembled. Trying to diagnose the failure. I bought it in running in a car this way. I have no history of what happened to it. It was running with three down holes, and it had an F1 procharger on it.

Getting it apart we found three broke piston top rings and lands. Sent the block to my local machine shop. They measured the bores to be from 4.310 to 4.312. I measured a piston to be 4.3" using my caliper. Not exact but close. Piston is probably a 4.310 which is a standard size for my rod and stroke combo making it 526ci.

The engine is using a 7.1" rod, 4.5" stroke crank. With 4.310" piston it works out to be a 526ci engine. This brings me to think about piston to wall clearence. Should have about .005", is that correct? With a piston measuring 4.3", and the bores at 4.310 to 4.312. This engine had no chance of survival. Or my measurement could be off. I dont know who built it.

If you had broken piston top rings & lands I’ll wager the engine was not setup for boost !

You have to provide extra clearance of ring gap IF you’re gonna run boost ! The turbo guys know this too ….boost = Extra heat =
More Thermal Expansion….

You’re cylinder pressure increased way beyond normal , which due to the extra heat caused the rings to exceed the preset gap & butt against each other giving you the result you’ve unfortunately experienced!

Boost requires more end gap . How Much ?
You Decide ! How Much Boost ! Fuel Type !

 

[rickseeman]

I'll try to be nice. A 526 Hemi with a Procharger isn't an engine that you can measure the piston clearance with a dial caliper. You are going to have to go to get a micrometer and find out where you are at. Building an engine like this isn't easy. A hundred things come into play like how much boost are you running. If the bores are really 4.310-4.312 you might get by with new pistons. But you probably need to hone it to see where it cleans up at. This won't be easy.

 

[dvw]

Measuring with calipers isn't very acurate. You would be better off sticking a feeler gauge between the skirt and the bore. That'll give you an idea. The two causes that I would look for is detonation and the rings butting due to insufficient gap. Lets see close up pictures of the ends of the rings and the ring grooves.
Doug

 

[Paul_G]

You guys are all correct about the boost. I am rebuiulding this Hemi for NA. It is going in a different car when rebuilt.

What I know about how it failed. Blame it on the tune, yes. They did pull timing, it was at 26°. They did not tune the carb. Not even an AFR guage in the car. AFR at WOT was an engine killing 14+. The car has a water meth system. It will add methanol fuel to enrich the mixture some, and the water helps lower cumbustion temp, but not going to enrich by 4 full numbers.

The 440 that is in that car now with the Procharger making 10PSI max runs AFR's around mid 11's at WOT. Yes, the ring gaps are wide, set up for boosted application on the ring gap chart.

If your wondering, I got the car at a good price because of the damaged Hemi.

 

[rickseeman]

So if you are going to put it together as a NA 526 it should be a normal rebuild. Find out what the bores will clean up at. (I assume you want to start the rebuild with a fresh bore.) Then buy pistons to fit, and assemble as desired. What killed it before won't matter.

 

[RemCharger]

Boost turns the FUN pedal into the FAFO pedal.

 

[Geoff 2]

Pistons are not round at room temp. The idea is they become round when at operating temp. Different piston companies use different 'cam' grinds on their pistons, resulting in different piston to bore clearance. Also, 2618 & 4032 are the common alum alloys used for pistons, with 2618 having a greater expansion rate....& needing more bore clearance.