Hey all,



have been having some huge issues with a motor i'm building... have rebuilt it 2x now and have had the same problem both times.. Problem is that the rear rotor gear land is somehow coming into contact with the rear plate and chewing the **** out of it, hence making the oil control rings ineffective making a hidiously smokey motor and damaging the plate, i've had it machined once already.. and am going to have to do it again...



Motor had a meticulous rebuild, all bearings replaced, everything spec'd out...but still this problem occurs...



any thoughts on what could be causing this to happen? I first though that maybe the rotor gear itself had walked out, but no, no i checked that and its within tolerance.

so u have specced clearance of over 0.08mm from rotor edge to gear land and sufficient clearance from rotor to end plate? have u measured it with a descent micrometer?



do u have descent side seal clearance? they arent forcing themselves against the plate when hot?



have u got pics?

A common outcome from overheated oil and or, coolant. you can also do it by just missing a shift and screaming the engine. So a rev limiter is a good insurance policy. The gear walks out a bit and the show is over. Anytime you have a rotor out of an engine, slip it in the press and give it a good push. 9 times out of 10 it will move a bit. That is why there is a snap ring option from Racing Beat and others. You can also drill and tap 6 little holes and screw in socket head set screws that engage the gear and the rotor, after you have pushed it in good and hard. I grind the gear land and the opposite land down to the minimum height.



You might also check the oil spray hole and nozzles for a partial blockage. A crank from an older car can collect a lot of debris and varnish internally like a centrifugal separator. You might also check the rotor bearing surface on the crank for a tiny amount of taper.



Generally the nitride is only .003" deep in the iron, and one lapping is about it. There is only a need for nitride for long wear life. So be sure to have the iron redone, or just start again with a new iron.

This motor has only been started up a few times and run for only a few minutes before the problem became obvious again. So no over heating of the oil or coolent eaither. Oil sprayers and nozzels have all been checked and blown out with compressed air. I went kinda OCD when doing this build...



Will check the shaft again, but I didnt see detect any taper on the shaft bearing surfaces initially...



All other clearances are perfect, all other parts of the motor are undamaged, except for the rear plate which makes the culprit the rear rotor. When i tore it down the first time, you could see bits of cast iron from the plate stuck to the gear land.



Guess ill be ripping it down again and as you say Lynn, chucking both rotors in the press and then grinding down the lands to the mininum height. Then get the plate machined again...



any other tips regarding this problem?...motor is a 12a Bridgeport Rx3 block with original RX3 rotors.



Cheers

If the rotor still looks good. Measure it with a height gage or calipers and find the thickness in three places, just below the corner seal holes. Then with a straight edge masure the height of the land area to that same place below the corner seal hole.

My bet is the gear is out a bit. There is no other explaination. Perhaps debris is between the gear and the rotor now.



For a failure to take place at idle just screams that a fault was in place before assembly. It is OK to touch the lands against the irons now and then, usually under very hard braking, or upshifting a dog ring trans. But the black death seen in so many cases is just plain old "rotor too thick for the space" problem. The steel gear against the iron is just not a good bearing match. A shorter land and 6 Teflon buttons would be way better, but the lands just about never touch. The springs in the side seals, corner seals and oil scrapers do a great job keeping the rotor centered between the irons. If you were planning on shifting above 9,000 RPM, then the rotor should be shaved down about .010" on each side from the outer oil scraper groove. For over 10,000 RPM then I would go .012". Bearings should have .0035" to .0040" clearance.



Lynn E. Hanover

Cheers Lynn, legend.

Just a quick question...as per rx3 FSM, torque specs for through bolts on Rx3 blocks (12a) is 24 ft/lbs... how critical is this and could an over tourqued motor cause this chewing of the rear plate...? say 30 ft/lbs?



Have had the plate machined and have pressed the rotors in the press, and are just about to re-assemble the block when i got thinking...



Also how common is it for rotor housings to shrink/warp? as im thinking that a slightly thinner rear housing would cause the gear land to run closer to the plate surface, and when heated, may expand and touch.

It can happen, usually the chrome gets flaked all to hell when it happens though. Did you measure the rotor housings?



The heat expansion is not going to be really high, if anything the aluminum will try to expand more, within the constraints of the tension bolts. Get the engine hot enough and the tension bolts will win and the aluminum will crush. I am kind of a nut, I slightly undertorque my engines. The last 13B I did, I think I torqued to 22ft-lb. The benefit is that I haven't hurt the rotor housings from overheating, either.

Housings both measured a clean 70mm in several places, tourqued the motor to 24ft/lbs and everything seems sweet. Guess will just have to wait and see on start up.

if the rotor housings measured 70mm what did the rotors measure?



the housings do shrink a bit, the thrichoid surface sags in the middle. at 70mm it sounds like they are good though



i dont really trust my torque wrench, so i shoot for the center of the range, which is more like 25lbs/ft



i have done a studded motor, and torqued it up to 55lbs/ft with no issues other than the owner, run an air filter.