I'm sure Mr Adkins has developed his information after exploding a large number of engines. And he is a drag racer. There are several other explainations for the sheared dowel, or cracked out (usually upper) dowel hole. The engine is trying to turn in opposition to the drive shaft, with the same amount of force, (Newton). The opposing load is carried away by the motor mounts. If you still use the front mount, look for the studs to crack out their holes and start leaking.   So, the engine is in torsion its full length and like a deck of cards between your palms when you twist your hands in opposite directions the pieces try to displace a bit each. Only the friction of the clamping force, of the case bolts and the four dowels resist this twist.

 

Over the years Mazda moved the engine mounts to the center iron, so some of the twisting load was removed from the front half of the engine.  Many years ago I was looking at the National Champion RX-7. It had a Pport 12A engine, and the chassis was superbly designed. The engine was suspended from two rods in the vertical from the fire wall, to two tabs on the bell housing. I didn't discover the thinking on that until years later. It was to take out the twisting load from the stack. The internal loading is still there, but the resistance to it is all in the rear iron. Now if that little hp hop up up was enough to break a dowel hole, the engine would have no supporters at all. So that failure must be unusual. It is becoming more of a problem. because the kids now run the boost up to 30 pounds and pour on the nitrus, and every now and then they have to pay the extra $50.00 to clean up the oil off of the the dyno table. And more to tow the car home.

 

As the HP went up Mazda has added material around the dowel holes to help with the problem.  So, FD irons (twin turbo 93-95 models) have the stiffest irons. The dyno racers and drag racers also add oversized case studs instead of bolts. The stack bolt holes must be reamed to fit the studs. On the combustion side, they add additional dowel holes and dowels, with stock case bolts, to keep the hot side from flexing under high boost. I have also seen external girdles with a screw jack pushing in between the spark plug holes.

You should be able to see the black mating surface on the housings where they scrub on the irons under higk pressure. It is normal, and even goes on when the engine heats up a bit. Compared to the irons the housings can look like they are made of hard rubber. They are very flexible.

 

Things to do now?

 

I open the Rotary Bible (Racing Beat catalogue) and see that the case bolts should be torqued to 32 pounds for high output engines. I would go one step further and say, use nickle anti sieze compound on the threads and under the bolt head and under the sealing washer, and torque to 26-28 pounds. The torque in any manual is for clean dry threads. No two threads in a hole are the same, so actual bolt tension is all over the place with stock parts. Another reason for studs. You get rolled threads and a great aircraft nut to torque with inn the cast iron. The studs have a socket hex feature to screw them in with. Do the torque pattern 8 times. For drag racing I would retorque the stack every other weekend. That will help with the twisting. Hanging the engine from the center or rear iron will help as well. Put a spiral bead of silicone on each bolt so it won't sing as you pass through 8,500 RPM.

 

All of this stuff is what you do when you go nuts on the boost. Look for boost spikes in your data recovery. So whey did the iron break out? Probably not enough torque being made to do it. Probably a single misfire, or cross fire. Rear housing fired 180 out of phase. That will do the dowel hole in one attempt.

 

So, Mount the MSDs on opposite sides of the engine. Run a crank trigger or similar with the primary wires run inside a dash 4 by teflon hose with the stainless braid grounded. One hose for each set of primaries. Never together in the same hose. (That cost me an engine) Same for the wires from the MSD to the coils. Run inside dash 4 and ground. 

 

I would put the double ended MSD coil right on the engine with one wire from each end to both pliugs on one housing. One coil for front housing and one for rear housing. Plugs fire together of course. Keep the inductive plug wires as short as is possible.  

 

I don't think quickly heating the housings has anything to do with this. I actually have never heard that one.

 

The advance mapping must pull the timing down to an area within 10 deegrees of TDC. Maybe down to zero if its too hot or too cool. The engine should run well rich of peak EGT. Best power will be just rich of peak, but max heat will also happen there.

 

If the knock sensor says its detonating, try a pass with the trailing plugs shorted out. Detonation is charge temperature dependant,

and you have to run a bunch of intercooler to stay out of it. Spray water on the intercooler and oil radiator before each pass. Or pack them in dry ice.

 

With ceramic apex seals it can survive a number of detonation events. It will not survive one preignition event.

 

One engine mount as a mono ball and the other in rubber with a calibrated strain gage across it, and you data collection and you have a dyno.

 

If you are running a dog ring trans and shifting without the clutch, you must shim up the pressure plate to allow some slipage of the disc on gear changes. Those shock loads appear inside the engine as torque spikes that can shear off a dowel.

 

This advice may be worth what it is costing you.

 

Lynn E. Hanover

 

I would like to add something here. Last weekend we were running our supercharged RX3 at the drag strip. After a pass oil was everywhere. We cracked the rear iron at the dowel pin. Never seen this before and after doing some searching Dave Atkins thought we didnt get enough heat in the motor before we nailed it. We cool it way down before each pass. Were running about 17lbs of boost. Dave says were creating too much heat too quick and the aluminum is expanding so fast its cracking the iron. Were not over heating, but it does get there fast. New experience for me, thought I would pass it on. Could be important for you turbo guys.

Bob Mears