WE HAVE MANY VERSIONS OF THE CRAB AVAILABLE, BUT PREFER THE STOCK UNIT!!!
Crab Vacuum Brake Details:
The most misunderstood part of a 1942-1948 Crab style distributor is the "vacuum brake". Often called by many a vacuum advance , with that not really being a correct name.
The vacuum piston is hooked to manifold vacuum and has a piston with leather wear washer glued to the end. This leather washer drags on a advance ring inside the distributor causing the timing to slightly retard when vacuum is lower than spring setting.
This should always be used even when the vacuum is not hooked up as it steadies the spark advance.
When removed the advance jerk or jumps around quite a bit.
Note : Typically we ship the crab set at three turns inward on the spring for a light tension on advance ring. Ford suggests driving the vehicle with vacuum hooked up and turn outward until spark knock occurs on a slight load with throttle. Then turn adjustment in until it stops.
After building over 1000 of these Crab units a visual inspection shows the actual problems the customer may have been experiencing.
The unit to the left shows that the customer had adjusted the slide timing all the way. (advanced)
This was more than likely based on a lack of performance problem, caused by point wear.
Total teardown shows some rust and corrision on advance unit which will be cleaned and relubed..
One of the secrets from the orginal Ford folks was the placement of a felt wick ( the worm)) placed inside the Crab main shaft. This wick was lubed and allowed a slight amount of oil to transfer to the crabs outter bushing.
It must have worked very well as you seldom see a bad bushing in these units.
The 11A advance unit is also a very good part but many have had the spring leaves modified by bending etc in a effort to make the car run better in the olden hop up days. Actually a straight spring allows the advance to be all in at 2500 rpm or less so this modification is a "feel good" trick rather that a functional one!i
We clean and blast the point plate then paint with a heavy duty black primer based on my dislike of rust!
CS47 heavy duty contacts are serviced and used .We lube the fiber by soaking in marvel oil and lube the cam area with high temp grease for long life.
Often these contacts must be aligned with alignment tool to make them perfect!
All screws are replaced with stainless allen screws for long life as well.
Note that these contacts use a spring and a brass strap for a very good connection. Cheaper brands dont have the brass strap!!
Each unit is ran on a tester , adjusted and allowed to run for some time then readjusted. Purple nail polish is used at each adjustment point to help hold the screws and remind us its adjusted and ready to ship. New .25 mfd condensors are installed and tested as well. The right (drivers side) contacts control actual engine timing and the left side (passenger side) over laps to contol ignition coil build up or dwell.
Overall we give this distributor a AAA+ rating as it works very-very well !!!
Two versions of caps are used the early units used the black crab distributor cap and the later (46 up?) used the two piece rabbit ears cap. Housings are the same and the clips and bail must be used with later cap. Either part can be installed.
There is also a cast iron unit available that was used on some cars and truck. Personally i like this cast iron unit when painted black and hung on the front of a flathead.