vacuum brake.

[Lawrie]

Has any one paid any attention to the vacuum brake piston and bore clearances.
I see the ford specs are for a clearance of less than .001,
I see on my 33 there is way more than that ,more like .004.
I think this would cause a vacuum leak at the piston.
My job for the week is to set up the housing and bore it to a nice parallel bore and make a new piston with the correct clearances.
will keep you posted.
Lawrie

[ford38v8]

Laurie, I hate to disappoint you, but the piston clearance has little to do with anything important. It’s actually the condition of the leather that determines how much or how little vacuum is utilized in the vacuum brake system, and further, the spring tension behind the piston should be set only firmly enough to prevent radical advance/retard flopping of the advance weight wheel, in order to provide a retard condition only when the throttle is fully closed. This slight need for the retard is due to the vastly improved fuels available in modern times, as opposed to the old fuels that really needed the advance weight wheel to prevent engine knock.

[Lawrie]

Alan,
the piston has no seal on it ,and the little leather part only contacts the advance disc. so the seal for the vacuum is only the fit of the piston to the housing,
I made another piston today and repaired the bore and set it at.001 .seems fine on the engine .
I maybe working about nothing but if the piston is loose it will be sucking air on the two cylinders that supply the vacuum

Lawrie

[Bored&Stroked]

Hey Lawrie . . . I'm with you - the bore needs to be very clean, the piston needs to easily move, but it has to have a very close piston-to-bore fit - or you have a vacuum leak. With that said, I've never had the issue - but there is the first time for everything!

[GB SISSON]

Good info! I have a whistle from the distributor that goes away when the vacuum line is pulled. I have replaced the gasket twice to no avail.

[marko39]

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When it's all done check your engine vacuum to see if it increased if you know what it was before .

[supereal]

I must agree with my old friend Alan. The vacuum piston in the distributor was necessary years ago when gasoline was of questionable octane. The principle was that the vacuum would allow the leather face on the piston to be pushed into contact with the point plate by the adjustable tension of the spring and counteract the centrifugal force of the advance weights to lesson engine "knock" when intake manifold vacuum dropped under heavy loads. When we overhaul a distributor we back the adjust off as todays fuel will not cause a knock in normal drivimg.

[Ian NZ]

On my Ford Stroboscope I set the Vacuum piston to about 8lbs to pull the piston of the disc and I have found that most of the early type of early type distributors that have the aluminium bore that I have measured are worn therefor the screw has to be loosened off to the recommended setting more to give the piston less spring tension and the crab type with the bronze bush they can be set correctly. What Lawrie has done is correct by cleaning up the bore and making a new piston which I have done too to a distributor.

[Lawrie]

I know from doing lots of miles in our 33 that the vac brake needs to function if you want to have the optimal timing at light load cruise and also full throttle up a hill.even with 91 oct fuel.
My next job is to make a jig up to allow me to bore the housings to a nice bore and then make another piston same as I made yesterday, these I will fit to my spare distributors.
No one has mentioned the effect of the continuing vac leak past a leaking piston on the mixture of those two cylinders.
Lawrie

[Tinker]

I am following a issue with my truck. Vacuum advance was screwed in quite a bit. I was getting poor acceleration and bad performance underload. Took the vacuum plugger out almost to the end and it preformed much better, but not great. Still something going on. I'm sure it was running well retarded timing for a bit and wasn't well. Need to set the tbc and put a light on it. (37 motor)


Following this as I run the vacuum advance.

[rotorwrench]

I concur with Lawrie on this design. The engine will be more efficient on pulling hills and extra loads that it would without the automatic spark control. Sure it will run with minimal pinking but it will get better mileage with the load control.

I also agree that 0.001" is about where the piston would operate the smoothest. It may bind at 0.0005" clearance and it may have excessive manifold pressure loss if much larger than it needs to be to work smoothly. A little dab of oil in the piston bore should aid in sealing as well. It may get sucked out eventually but it should last a while anyway.

[Ian NZ]

Lawrie its a good idea to bore out the piston housing because I found they do not wear out evenly. In the old days when Ford reconditioned distributors the dealers would have these housings on the shelf as well as complete distributors
and if they were too worn they would have replaced them.

[Brian]

Just an idea, without looking at a distributor housing i don't know how much 'meat' there is to play with, but the cast iron crab distributor bodies have a bronze bush for the brake piston to operate in. Would it be possible to bore the housing and fit an oilite bush? Running an oil impregnated bush would improve the seal and last longer.

[Ian NZ]

Brian that would be a good idea then there be lubricant there all the time to seal it too.

[Bored&Stroked]

I have an NOS housing on the bench . . . will check it out and see how much "meat" is on the casting . . .

[Lawrie]

Good thought here , the bush seems a good one.
Lawrie