Race Engine Forum UK

This is more of an FYI for those of you that race your twin cam with out the belt guard. This what happen when a little stone finds its way into the system. Ouch! I just happen to go off track in the qualifying session and slid on the wet morning grass into the gravel trap. No big deal, I went in sideways, no stones that i could see in the engine bay, cranked it over and started right up. When i got back to the paddock i did a what i thought was a good cleaning of the underside pulling out misc. stones here and there. But come race time, in lap three in a long left hand sweeper before the front straight, all mayhem broke loose. The engine roared and lost power and when i pushed in the clutch it continued to rev up, what the heck! What happen is the intake valve broke and pieces of the piston blew back up the intake sticking open the idfs. I did a mad scramble to shut it down. Here are the photo of the damage.
Now for the question i have, take a look at the con rod bearing. They are Clevite 0.010" over size bearings due to a reground crank (1800 motor), they are missing chucks of bearing material on the cap end? No heat marks and the closest thing i could find to compare to on Clevite't site is Cavitation damage. The Mains were perfect, look brand new and they should this engine only has about 10-12 running hours on it racing mostly 4000-7500 rev limited.
What do you think?

Ted

Interesting failure-mode post Ted - but in future please use the site upload facility not photobucket as per Protocol (3).

The bearing failure is the result of local overload from pressure and heat applied to the aluminium/tin bearing material. You have direct metal-metal contact leading to brittle fracture and tearing. It is accompanied by heavy rub in areas not as highly loaded during the firing stroke as the fracture zone and has been going on for some time. It is a common failure mode in all rod bearing materials used outside the 'design envelope' and is the result of all or any of the following:

1. Overheating oil (85 deg C is the optimum on them)
2. Badly aerated oil or oil starvation (pump is pumping air not oil)
3. Contaminated oil (by gasoline - worn rings/over-rich/ex back-pressure).

I think potentially you might need some guidance on the oil system layout.

G

Thanks Guy,

Photos corrected, now that I found the up load button.

Is it possible the bearing damage could occur if the oil was too cold and subjected to high engine speed and load? I find it strange that there are no high heat indicators on the bearing shells or rods. Plus my Accusump is direct acting and plumed into the right side main oil gallery. The oil could still be aerated but pressure drops are not being seen on my gauges. This might be a good time to install an oil temp gauge, thermostat and look at my oil cooler take off adaptor for improvements.


Ted

We can 'kick this one around' a bit, Ted, no worries!

Ignoring your last - quick question: did you unplug and clean the crank oilways after grind?

G

I can only speculate that the crank was cleaned properly as I was not the builder, the rebuilt engine came with the car but from my inspection it does indicated that the plugs were removed and replaced. The main bearing shows no signs of contaminations and the regrind looks first rate. The mains and rod pin measure out on the low limit though for 0.010" undersize but with out new bearing and plastic gauge, accurate clearance measurements are only speculative at this point. Is it possible that oversized rod bearing material have a lower fatigue life?

Ted

OK, moving along.

Post high def photos of the head gasket showing each fire-ring region also shots of combustion chambers that are still intact.

Photo clarity vital.

G

Ok, Will try again.

Regards

Ted

Please delete and try again.

You are too close to the subject and the photos are too grainy or out of focus to read - maybe shutter speed too slow or inadequate light. Or maybe distance setting is wrong. I take high res photos on macro setting 3m distance and zoom in from about 1m away. You need to capture more surrounding regions not just little bits in the corner - I cannot get any idea of where I am in one or two photos.

Let me see a photo of all 4 cylinders - I am trying to read the surface condition of each compared with the other, likewise gasket fire rings all in line and close-up of any that show any sign of distortion esp in cylinders that do NOT have valve damage.

Below is the kind of quality I need, actually a smaller size than yours but far greater clarity.

G

Lost interest Ted?

G

Sorry Guy, seems that work is getting in the way. I am now on my way to Argintina for three weeks to support the start up of a new engine line for Ford.

Ted