Thread: Burned slugs 421 SB Stroker

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I read the article on compression, very informative. I however recently assisted my father-in-law in putting together a stump pulling 421 stroker. Well all was fine until the blow by showed up. What we found were No 4 & 8 pistons burned. Now we are not pro engine builders, but definitely not novice either. Been doing this for some time now. In fact I am currently running a 347 stroker in one of my Mustangs. The timing and everthing are within the allowable specs for the cam. I was just shocked to find 2 pistons burned like this. This engine has less than 5000 miles on it.

750 Holley and 104 octane fuel.
Lunati parts.
Any insight will be greatly appreciated.

Since both burned pistons were at the back of the motor, it's possible it was a fuel distribution problem causing those cylinders to run lean. Could also be an air/vacuum leak. What fuel system are you running? How did the plugs and combustion chambers of the other cylinders look?

How is the motor used? Did the blow by come on suddenly or was it over time? I guess I am asking if it could be tied to a single event or usage.

Pat

The problem started after a pass. I chucked it up to rings not seated properly or some other crap. It then got prgressivly worse. He made another pass a week later and it became unbearable. The car is used primarily for street fighting. In fact the mileage is probably more like 2000. I did not see any evidence of a leaking intake gasket. And since the two cylinders were not directly beside each other I don't suspect a head gasket. But anything is possible. We are using the Summit electric fuel pump kit, with a "team G" intake. The other cylinders were fairly dark. I have not check the plugs yet.

Roy

This is a Ford Windsor right? So #4 and #8 are the back cylinders? What style of manifold is the Team G? Single plane or dual plane?

How was the weather the day things went bad, was the air cool and dry? A little north breeze? In other words lots of oxygen?

Pat

No, Sorry I misled you, this is a 400 chevy SB stroked to 421.
It was a cool night though.

Hey Roy;
Sorry to hear.

My first question is, did you have the motor torque plate honed? Just in case you are not familiar with that term, this process is done after an engine is bored, the machinist takes a steel plate about two inches thick and bolts it on top of the bare block. This plate has holes for each cylinder as well as bolt holes in the plate. This process is done to simulate the stress applied to each bolt hole after the heads will be torqued in place. After this plate is properly torqued on the block, you leave it over night in order for the bolt holes to stretch the metal inwards towards each cylinder hole.

The next morning the final, about .001 -.002 of an inch is removed during a honing process, not boring. At this point there will be 5 high spots in each of those cylinders due to the pressure of the bolts’ forcing the metal towards the open cylinder holes of the block.

Now, a lot of machinists have no idea that this process is an absolute must for the sbc and in fact, most machine shops do not even own a torque plate. If this is the case of your shop, find a new shop, no use trying to educate or tell the professional anything, they do not listen, from my experience.

Having said all that, check the block to see if there are cross hatches in each cylinder, hopefully your man at least did that right. Finally do not re-use rings, even if they are only one week in a block.

Roy, did you check ring end gap on all 16 rings? How much did you gap the upper ring and lower ring at?

Do you have four corner cooling?

Did you cement the block partially?

What do you mean by “pistons burned”?

What are the last 3 numbers of your block?

Roy,

I am out of ideas. Pretty clearly something caused those two cylinders to run lean. You might want to contact the Weiand people to see if they have heard of it before. With an open plenum and non adjacent cylinders it's tough to think jetting would be the cause. Those cylinders are consecutive in the firing order, but I can't think of what impact that would have.

I mentioned the cool dry weather because I have seen a bunch of carburated two stroke boat motors burn pistons on days like that, especially in the fall.

Maybe some of the other wise men will have some ideas. If you figure anything out please let us know. Good luck.

Pat

Nice try streets, you are not "up here", you are down there. hehehe! In the cataloque they are under, Torque plates.

I used pre-cut rings the first time and failed to check end gap. Also used a new machine shop to keep my rivals from knowing we were pulling an engine swap. Went from a 327 to this new motor. So to answer your questions,

 No the block is not cemented at all.
 The last 3 numbers are 511.
 By burned piston I mean that they have they both have an edge of the piston burned off making the top ring visible.
 And what do you mean by 4 corner cooling. The intake we’re using is blanked in the rear. No crossover ports for the rear of the heads.
 How do I achieve 4 corner cooling.

We will be going back with file fit rings this time. I rocked the pistons from side to side and the bore seems a little loose.
What should the actual bore be for .030 over slugs?
Any help would be appreciated.
Constructive criticism welcomed.

Roy

Using “pre-cut” rings is fine, however they should always be checked for ring end gap. File fit rings come way over size and a lot of filing has to be done. My first guess is that your gaps were less than .020 for your top ring and so when the engine warmed up and the rings expanded under full throttle, the ends butted together and knocked off the top ribs on two pistons. My second guess would be that you experienced pre-detonation and this caused the top edges to blow off. Finally, you might have had too much timing in the ignition and rattled the pistons apart.

I've built a couple of 3.875 strokers both a 415 and 421.

I've seen pistons broken like you described. They were in race motors that experienced a lean out or way to much spark advance which causes detonation which breaks the crown.

You mentioned 104 octane is this also a 12.5 compression or so motor?

My other question is what kind of heads are you running. Heads like my Dart 227cc only require only 32 degrees of advance to make max power.

The heads are an older Brodix set. They have been ported and resurfaced. After talking with you guys I think I may have been pushing the envelope on octane and timing. We were running 36 degrees of total timing. I don't know what series Brodix heads these are. According to the machine shop they were manufactured in 1980. The heads will be cc'd before they are put back on this time.

Again, any further advice or observations are welcomed..........

Thanks again
Roy

Older heads like your 1980's I don't think have the modern combustion chamber shape to incorporate high swirl/fast burn technology.

There are only a couple of things that can break pistons like that. One I didn't mention is lack of ring gap. Installing a top ring with little or no gap will cause them to seize and break.

We run methanol as fuel in our race cars. burnt pistons from leanout usually puts a hole right through the piston top. The whole cylinder - piston top chamber - spark plug looks like it's sprayed with powdered aluminum. There is no breaks or cracks.

Nitrous motors that hit into leanout/detonation have broken busted ring lands and crowns. Detonation in a normally aspirated motor is not normally as severe unless you were way out on timing or your using a MSD 6 series rev limiter

In fact I am using the 6AL with rev limit.............
What in the world is that doing to me?

Roy

I've been going to Bonneville Salt Flats for years and I also road race. Lot's of blown motors when your trying to run an engine at WOT for miles.

So many years ago I show up with a 6AL MSD and the tech inspector said that I would be better off removing it. So Blah blah blah...............

Those style of rev limiters use random miss fire to limit motor rpm. So you could have one or two cylinders missfire a few times in a row filling the pipe and cylinder with raw gasoline while the other cylinders are still under max power. Then it fires the raw gas cylinders or even the raw gas pumping down the header pipe with an open exhaust valve catches on fire and boom - busted piston crowns .............

No real race cars use those old random missfire units or crap with pluggin chips. I use the Crane digital H-6R It just makes an odd lite rattling sound when I'm petal too the metal before a turn and braking on the road racing tracks. I don't even worry driving it into the rev limiter. Most race cars do it some where on the track because you can't have perfect gearing for every turn.

I can remember big backfires and popping sounds when I had the 6AL junk