The Road to Drag Week 2016 - July 17, 2016

[jayb]

And another thought occurred to me after talking with a friend of mine today.  Back in 2014 when the rod let go, I disassembled the engine and found the ding in the combustion chamber mentioned a couple posts back.  I assumed that this was due to the piston flying up and hitting the chamber after the rod let go.

But what if the ding in the chamber was due to the piston repeatedly striking the head at high RPM?  If I had piston dome to chamber interference for some reason on that cylinder, that would explain why the first rod failed in 2014, when I had been running it successfully in the engine since 2008.  In 2014 I put on a new set of heads, caused the piston to chamber interference because of some issue with either the piston or the chamber, and broke the rod.  Then, put it all back together again for last year, and the same thing started to happen.

What's more, the ding is on the front side of the chamber, so a fore-aft crack in the rod makes sense under this scenario.  This is actually the first really rational explanation for the problem, I think.  I can test it by installing last year's head on the short block when I get the short block together, and clay the piston top to check for clearance.  Should be enlightening...

[JERICOGTX]

Why not mount it with a motor plate?

[jayb]

Why bother?  A motor plate would add complexity and cost, cramp the already cramped engine compartment, and the factory mounting points would still be sitting there.  The Modified class requires stock front frame rails.

[WConley]

Jay - The head whacking the piston makes a lot of sense.  An off-center hit could be fatiguing that rod.

Years ago a friend rebuilt his 240-Z engine and decked the block to clean it up.  At high revs the engine made a strange ringing sound that we couldn't figure out.  Finally the head came off and all of the pistons had witness marks from smacking the quench areas of the chambers.  Statically he had enough clearance, but stuff was stretching when he zinged it.

Interesting stuff!  Glad I'm not writing the checks.

[machoneman]

And another thought occurred to me after talking with a friend of mine today.  Back in 2014 when the rod let go, I disassembled the engine and found the ding in the combustion chamber mentioned a couple posts back.  I assumed that this was due to the piston flying up and hitting the chamber after the rod let go.

But what if the ding in the chamber was due to the piston repeatedly striking the head at high RPM?  If I had piston dome to chamber interference for some reason on that cylinder, that would explain why the first rod failed in 2014, when I had been running it successfully in the engine since 2008.  In 2014 I put on a new set of heads, caused the piston to chamber interference because of some issue with either the piston or the chamber, and broke the rod.  Then, put it all back together again for last year, and the same thing started to happen.

What's more, the ding is on the front side of the chamber, so a fore-aft crack in the rod makes sense under this scenario.  This is actually the first really rational explanation for the problem, I think.  I can test it by installing last year's head on the short block when I get the short block together, and clay the piston top to check for clearance.  Should be enlightening...

The plot thickens! Sounds most plausible (fore-aft cracks). What is the nominal head-to-piston clearance that you run? Heavy piston+heavy pin+lotsa rpms+ lotsa hp = rod stretch.

[Barry_R]

I am signing in on this theory - best guess so far. 

When I built the SOHC that I intended to run in EMC I had the chambers digitized at Diamond and tried to create a +/- 1/4" wide quench band around the outside diameter by bringing that surface within .010 of the chamber and having a tiny degree of angle tilted toward the center before adding in the gasket.

My initial test assembly with a gasket showed it to be too close - touched the head - and ended up tweaking things just a bit.  With the radius it left no marks at all - just kind of snuggled up into the chamber & pushed against the head.  If you were close but not hitting hard you would never feel a thing turning the engine over.

[cjshaker]

If that were the case, I'd think there would be some other tell-tale sign that was specific to that cylinder. Different markings on the piston skirts, the cylinder, the chamber, the rod bearing or something. If it's shocking the rod enough to cause it to crack, I just can't see it not leaving any other mark. Being nearly all aluminum, the rod is the strongest part. Granted, it's also the most brittle, but aluminum is much more malleable so I'd think there would be some sort of deformation somewhere.

I'll stop being a Debbie Downer now.

[jayb]

Picked up the pistons and replacement rod today.  I had the piston pin bores checked for roundness, and they were perfect; no re-honing required.  Sometimes if you have a serious detonation problem the pin bores will go egg shaped, but these pistons did not have that issue.  If they had been out of round, it would have been another piece of evidence that the engine encountered severe detonation, and that could help explain the movement of the oil ring support rail.  But, they looked fine.  I still don't have an explanation for the oil ring rail issue.

Also, when I picked up the pistons I gave #5 a very, very close look.  There is zero evidence on the dome that it was hitting the chamber.  Maybe it still was, and the cast aluminum chamber was weaker and gave in, but I would have thought I could see at least some contact evidence if I looked closely at the top of the piston.  There isn't any.  Guess I won't know for sure until I clay it. 

At least now that I have all the pistons back, and the replacement rod, I can get the short block finished up over the next few days...

[RoyceP]

Let's say the engine puts 400 Lb - ft of torque on that strap. I think you may be onto something here Jay. Perhaps increase the clearance on #5 piston to head, and add the other strap.

Now there's an interesting idea.  Seems a little far fetched, but who knows.  The engine is mounted with factory mounts, and the torque strap is mounted right up front on the driver's side, to one of the #5 exhaust bolt holes.  Hmmm, maybe I should be putting a matching torque strap at the back of the block...

[Royce]

As unlikely as it seems, I think you have to consider the possibility that the rod cracked due to an internal flaw or metallurgic deficiency.. I would send that rod to someone for failure analysis..
 Somebody said that "once you eliminate the likely possibilities you are left with the improbable or seemingly impossible as the truth".

[jayb]

As unlikely as it seems, I think you have to consider the possibility that the rod cracked due to an internal flaw or metallurgic deficiency.. I would send that rod to someone for failure analysis..
 Somebody said that "once you eliminate the likely possibilities you are left with the improbable or seemingly impossible as the truth".

That was the conclusion the last time.  We thought that in 2014 the rod broke due to an internal flaw.  Now, here it is again, one year later.  Same rod, crack in the same area.  I'm not buying the two defective rods in a row theory; it is just too coincidental.  I'm certain at this point that it is some problem with the engine, and now I am equally certain that the failure in 2014 was not due to a rod defect, despite me (and nearly everyone else) thinking that it was.

[jayb]

Let's say the engine puts 400 Lb - ft of torque on that strap. I think you may be onto something here Jay. Perhaps increase the clearance on #5 piston to head, and add the other strap.

Definitely going to add a second torque strap this year, Royce...

[machoneman]

Good to hear on the 2nd strap.

Jay, does the tranny have a rubber mount under it or a solid metal mount? Just asking....as I used solid engine mounts AND a solid tranny mount long ago in my old doorslammer 10 second BBC '69 Camaro. Big  mistake as I broke a few ST-10 cast iron tranny cases before I realized chassis twist was the real culprit.

[jayb]

Rubber mount on the trans also.  This is a STREET car 

[machoneman]

Hah!