Scroll clearance

[Bill Traill]

Charlie, The clearance for the scroll (in my opinion) is .004 to .006inch. I used to work in an automotive machine shop specializing in antique automobile engines. The shop owner said .004 to .010inch for Model A Fords, but I am skeptical of the higher number for the XPAG. Scroll clearance too close (capillary action), or too far, will leak. If the scroll or cap show wear (like if the crank had been slightly bent and the scroll/cap wore on one side, or rear bearing was once wiped out) it will leak. If the scroll is roughed up, it will leak. Check the scroll, bearing cap and alloy half for handling damage, dinks, doinks and dents (machine shop terms). In these cases the scroll will have to be trued up, the rear bearing cap repaired and / or milled down and line-bored (with alloy half) for bearing and scroll diameters. I have checked my XPAG's scroll clearance several times (3 or 4) in the past 37 years at every engine o'haul (usually just new piston rings). Always success for a while, no rear seal dripping, until after 5000 / 10,000 miles. As the engine ages it will become incontinent. Blow-by will increase as the piston rings wear and the blow-by shock-wave will push the oil past the scroll at idle. Clutch rotation in the bell housing draws in air, and the bell housing becomes pressurized. I think that was design intent to keep the oil in the engine at road speed. Proof...turn the 'inspection' plate, top to bottom (vent slot reversed), and oil will be sucked out of the engine and maybe the gearbox too if the leather seal is near warn. Latest o'haul in 1999 (25,000miles), my XPAG would not drip if the rpm was above 2000, but let it idle for a short time and there would be 20 drops of oil per minute. Several broken piston rings were replaced. Scroll clearance and bearing/crankshaft clearance were spot-on. These broken rings are something I haven't figured out. AND.... I suggest you install a small block Chevy rear seal as per Neil Bradley's article in the T-ABC tech section. It will cut down 98% of the oil puddle on the garage floor after the engine has aged. Cut down on the amount of oily dirt sticking to the chassis too. Don't worry, the tradition of oil spots under your TC will always be with you...valve cover, tappet inspection cover, porosity of the sump, leaky oil line, transmission, diff etc. Porosity of the sump! Something I will have to ask my doctor about next visit. Bill Traill TC 5221 Santa Clara, California, USA