Unbelievable engine survival main cap break

[340king]

These photos are from a small block Chevy engine I am advising on. This is the center main of a 4 bolt block. The cap has a ductile fracture just outside of one of the main bolts. There’s absolutely no evidence of fatigue in the fracture mating surfaces.

My working theory is that the cap was fractured during assembly when one side was tightened too much before the other side dropped into the register. The engine raced approximately 70 nights in this configuration as a USRA Bmod. The bearing shows that only part of it was loaded. The last night of competition this engine placed second in the A main!

The interesting part is that one of the adjacent cylinders showed more wear than the others. I assume this was due to less oil spilling out of the top of the bearing since there was this gap at the bottom. The adjacent rod bearings look good enough to put right back into service however.

I never would have imagined that something like this could happen. Please give me other plausible reasons how/why this happened if you have something you have seen like this before.

[Schurkey]

Inclusion (slag or other contaminant, "dirty" iron) in the iron the cap was poured from? You'd think they'd have heard the cap pop if it cracked during engine assembly. (Maybe they did, and ignored it?)

Oldsmobiles are known for breaking the #4 cap. My understanding is that the 18436572 firing order makes life difficult for #4 main. which is why Ford went to the other firing order for the 351, and why GM changed the firing order when they built the LS-series. But the cap shown is for #3--right?


Many modern vehicles advertise "fracture-split" connecting rods. You're just ahead of the curve...

[Ken_Parkman]

Long time ago took apart a Nitrous BBC from a Q16 dragster application and one of the caps came off just like that. The rest of the engine looked fine. No idea when/how it broke. Does make you wonder.

[bobalattie]

351w, pulled the oil pan to replace the oil pan gasket. Found this. It had a season and a half in a hobby stock.. Everything looked great, but had to have a new block prepped again. Broke the center main cap which is also the thrust cap..

[340king]

Wow, we couldn’t see the broken cap as the crack was closed up tight in the assembled engine. This wasn’t a gorilla of an engine as the pistons were 0.050” below deck. We are going to fix that along with the main cap. I just finished a rebuild of his brother’s USRA Bmod engine which prompted this advisory request. I also think I will be tasked with assembly. Hopefully I don’t mess up a cap. It had studs in it which might have caused some of the issue since it’s a bit easier to hang the cap on the register with studs.

[340king]

Inclusion (slag or other contaminant, "dirty" iron) in the iron the cap was poured from? You'd think they'd have heard the cap pop if it cracked during engine assembly. (Maybe they did, and ignored it?)

Oldsmobiles are known for breaking the #4 cap. My understanding is that the 18436572 firing order makes life difficult for #4 main. which is why Ford went to the other firing order for the 351, and why GM changed the firing order when they built the LS-series. But the cap shown is for #3--right?


Many modern vehicles advertise "fracture-split" connecting rods. You're just ahead of the curve...

I wondered about hearing the cap break also. The builder was an older machinist and might not have been able to distinguish the cap cracking from the cap dropping into the register.

This’ll be the first time I’ve ahead of the curve!

[Keith Morganstein]

These photos are from a small block Chevy engine I am advising on. This is the center main of a 4 bolt block. The cap has a ductile fracture just outside of one of the main bolts. There’s absolutely no evidence of fatigue in the fracture mating surfaces.

My working theory is that the cap was fractured during assembly when one side was tightened too much before the other side dropped into the register. The engine raced approximately 70 nights in this configuration as a USRA Bmod. The bearing shows that only part of it was loaded. The last night of competition this engine placed second in the A main!

The interesting part is that one of the adjacent cylinders showed more wear than the others. I assume this was due to less oil spilling out of the top of the bearing since there was this gap at the bottom. The adjacent rod bearings look good enough to put right back into service however.

I never would have imagined that something like this could happen. Please give me other plausible reasons how/why this happened if you have something you have seen like this before.

Letter K cap? I call those “ kaboom caps”, they break like that..

[ProPower engines]

Letter K cap? I call those “ kaboom caps”, they break like that.. I like that one Keith.

Yes I have seen many stock 4 bolt caps break in half like that in circle track engines and I was told 40+ years ago it was from crank whip.
While it looks like it was line honed and I hope the builder confirmed the registrars were square to the cap mating
surface before assembly but back when the 3.750 stroke 350 main cranks hit the markets back in the 80's to allow builders
to not have to turn the mains on a 400 crank to suit the block we saw that alot and thought the cranks were too flimsy as well but even seen the same deal with steel cranks stock GM or aftermarket.

But I have also seen the balancer weight be an issue as well in roundy round stuff from the crank winding up one way during acceleration then unwinding the other way when the driver lifts off the gas. Most time I have seen this is when the flywheel is very light but a stock heavy balancer was used on the same crank. Also causing the key to shear off and damaging the snout of the crank at the same time.
Some inline 6 cyl. stuff is bad for that as well. But the same reason a drive shaft swists up and breaks in the center of the tube at times a crank can see the same sort of twisting motion and break the cap and or the crank as well.


Best mag the shaft before re-use.

[In-Tech]

I think I posted about this before but we'll go again. I had a "604" crate engine apart and found a broken cap, couldn't see the crack but I wasn't really looking either. I truly believe it was broken during assembly. Bearing looked fine, not much hp crate engine, I forget what they are rated.

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[GARY C]

My engine builder jumped my a$$ when I was young because I was fixing to run the bolts down on a cap that was not seated in the register... He said, if you want to break a cap thats the easiest way to do it.

It's amazing how fragile such a strong part can be if it's stressed outside of it's design or visa versa.

[mag2555]

Long Dowels are the best way to locate main caps , not pinching it in at the parting line!

[ZIGGY]

X2 to Pro's comments about circle track engines though I'm certainly not in his league. Years before cell phone cameras, we broke the center out of a cap while racing. There was enough left to retain the bearing and the engine kept running at race speed. The first indication of trouble was some flame from a small oil fire where the cap chunk holed the pan. Before I could run to the flag stand, a lucky caution slowed things so I could wave the car in.
That claimer engine had about 30 nights on it, which was at our throwaway limit for stock cranks and rods. Monkey dusting showed the crank was cracked. The engine had a stock balancer and ran through an Ernieslide with very small clutch and stock automatic flywheel. I never considered the possible effects of that combination until now.

[1972ho]

Another case for JB will get it done.

[MadBill]

Byce's Block & Cam 014.JPG

The few broken caps I've personally inspected didn't show classic fatigue indicators; I think the ones in the thread have all happened on the track.

This shot is of a road race 1970 Boss 302" SBF engine making 630 HP*. The only symptom was a sudden 10 psi oil pressure drop. The cap showed no signs of fatigue beach marks. Note the perfect alignment of the cap and block breaks and the immaculate condition of the bearing.

* Yes, I know stock blocks are not supposed to survive with more than 500 HP. This one ran for many years with 485 HP and ~12 hrs. with 630...

[modok]

I think the ones in the thread have all happened on the track.

I think the same, from the looks of it. If it was broken for a long time I believe on the back of the bearing you'd see more fretting and less shmoo stain.
although, I can't really prove it.

This happens so rarely, I can't say much for sure.