BB ford distributor shaft to block clearance

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BB ford distributor shaft to block clearance

Post by RT Machine » Tue Feb 13, 2018 6:39 am

What is the correct clearance between the shaft extension below the distributor gear and block on BB and SB fords ?
As I understand it the hole in the block does not support the distributor shaft and it is supposed to be a clearance condition, is this correct ?

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Re: BB ford distributor shaft to block clearance

Post by Walter R. Malik » Tue Feb 13, 2018 9:30 am

Whether correct or not, I don't know but, I prefer .008" maximum.
The gear is supposed to ride on the shelf of that hole.
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Re: BB ford distributor shaft to block clearance

Post by Tuner » Tue Feb 13, 2018 12:28 pm

https://performanceparts.ford.com/downl ... lation.pdf

The downward thrust caused by gear tooth angle and the load of driving the oil pump pushes the gear down against the flange in the block. Measure from the upper flange on the block where the distributor housing seats to the flange down in the block where the gear sits. The measurement will be very close to 4.035" (FE 3.075") The distributor end play must straddle this dimension so the gear can touch the flange in the block and shaft is not in contact with the distributor housing in either direction. If the flange in the block is 4.035" the gear flange end play should be from something less such as 4.025" to something more such as 4.045" so it cannot bottom out on the housing in either direction, up or down, when the gear is in contact against the block.

Follow the Ford Motorsports instructions in the link posted above. The MSD instructions are wrong. If you measure your block you will see Ford knows what they are talking about and MSD must have pulled the numbers out of their ....... janitor's ear.

The FoMoCo blueprint specs are FE 3.071-3.078, all other V8s are 4.031-4.038. Your block will be in that range. Distributor shaft end play must be larger than this so the gear can touch the block with no contact between the shaft and distributor housing.

The shaft must be able to float in the middle of the end play range when the gear is pushed down (by the tooth pitch thrust from the load of turning the oil pump) in contact with the flange in the block, not in contact with the housing up or down. The Ford instructions describe it well. There must be slack for the shaft and slack for the advance mechanism.

I have only ever seen one block out of the range published by Ford and it was only a few thousandths. I recall it was a '70 429 CJ in a Torino.
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Re: BB ford distributor shaft to block clearance

Post by Paul Kane » Tue Feb 13, 2018 12:57 pm

.
It's technically known as "distributor gear thrust boss clearance" and is the most overlooked setup in these engines.


Image

The majority of people shearing their distributor gear roll pins have an incorrect thrust boss clearance, then blame it on something else such as their HV oil pump--duh. In reality, the distributor gear shears the pin downward (not rotationally) because the thrust clearance is set to high (ie, gear is riding too high on shaft and "floating"). The same setting will also prematurely wear bronze distributor gears against billet steel camshafts.

When set too low, the thrust boss in the block gets prematurely chewed up.
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Re: BB ford distributor shaft to block clearance

Post by RT Machine » Tue Feb 13, 2018 1:02 pm

Thanks for excellent info in your replies, all this is exactly what I was after =D>

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Re: BB ford distributor shaft to block clearance

Post by Walter R. Malik » Tue Feb 13, 2018 6:19 pm

Tuner wrote:
Tue Feb 13, 2018 12:28 pm
https://performanceparts.ford.com/downl ... lation.pdf

I have only ever seen one block out of the range published by Ford and it was only a few thousandths. I recall it was a '70 429 CJ in a Torino.
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Sometimes ... that distributor gear thrust flange in a well used block has some excessive wear and has a vertical dimension much more than 4.036" so, simply a hardened distributor flat washer can be used on top of that thrust surface to bring it back into the correct specification range.
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Re: BB ford distributor shaft to block clearance

Post by MadBill » Tue Feb 13, 2018 6:52 pm

Not a Ford guy, so this may be off base, but wouldn't it be easier to just mount a dial indicator on the distributor, contacting the top of the shaft, zero it with the shaft all the way down then bolt down the distributor, note the resulting reading and then lift the shaft to verify some further clearance?
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Re: BB ford distributor shaft to block clearance

Post by Walter R. Malik » Tue Feb 13, 2018 7:46 pm

MadBill wrote:
Tue Feb 13, 2018 6:52 pm
Not a Ford guy, so this may be off base, but wouldn't it be easier to just mount a dial indicator on the distributor, contacting the top of the shaft, zero it with the shaft all the way down then bolt down the distributor, note the resulting reading and then lift the shaft to verify some further clearance?
That could be done but, that would only verify that everything is already correct.
If there was no upward movement at all, you would still need to go back and measure everything.

Worse would be that the gear is down to far and the stop collar pinned to the shaft would bind in the housing at full UP position.
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Post by dwilliams » Tue Feb 13, 2018 8:09 pm

RT Machine wrote:
Tue Feb 13, 2018 6:39 am
What is the correct clearance between the shaft extension below the distributor gear and block on BB and SB fords ?
I never came across any official figures, so I used .0005 to .0015" and made some gauges on the lathe out of some round bar stock. I found a couple of blocks in the .005" range; I reamed those and drove bushings in.
As I understand it the hole in the block does not support the distributor shaft and it is supposed to be a clearance condition, is this correct ?
The hole in the block is what regulates the distance between the distributor gear and the cam gear. The bushing in the distributor is waaay up there. (and some of those are loosey-goosey, too, if you ever run into weird ignition troubles)

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Re: BB ford distributor shaft to block clearance

Post by NXBOY » Tue Feb 13, 2018 11:00 pm

I just used a MSD gear and their instructions and with the front cover off I got .009 clearance. It lays on block and only about .009 filler gauge goes in. I noticed long time ago MSD says meassure with end play pushed out. I researched this long time ago and believe Ford said with it pulled away. It was my conclusion that there assuming +.030 of end play in the shaft housing. Off the top of my head the long side of measurement was 4.000 with slack pushed together.
Last edited by NXBOY on Tue Feb 13, 2018 11:50 pm, edited 1 time in total.

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Re: BB ford distributor shaft to block clearance

Post by Walter R. Malik » Tue Feb 13, 2018 11:47 pm

NXBOY wrote:
Tue Feb 13, 2018 11:00 pm
I just used a MSD gear and their instructions and with the front cover off I got .009 clearance. It lays on block and only about .009 filler gauge goes in. I noticed long time ago MSD says meassure with end play pulled out. I researched this long time ago and believe Ford said with it pressed together. It was my conclusion that there assuming +.030 of end play in the shaft housing.
You've got that backwards ... 4.036" EXTENDED shaft ... MSD's 4.000" recommendation is with a compressed shaft.
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Re: BB ford distributor shaft to block clearance

Post by NXBOY » Tue Feb 13, 2018 11:51 pm

Was fixing as you text lol Randy

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Re: BB ford distributor shaft to block clearance

Post by Tuner » Wed Feb 14, 2018 9:07 am

RT Machine wrote:
Tue Feb 13, 2018 6:39 am
What is the correct clearance between the shaft extension below the distributor gear and block on BB and SB fords ?
As I understand it the hole in the block does not support the distributor shaft and it is supposed to be a clearance condition, is this correct ?
uh-oh #-o Reading comprehension fail ........ The OP's question is about the clearance of the OD of the shaft below the gear to the ID of the hole in the block ....... not the gear thrust flange installed dimension. I don't recall ever bothering to measure this or noticing a problem with it except de-burring the upper edge of the hole after a bronze gear installed too low on the shaft (too far from the flange on the distributor housing) spalled the thrust flange in the block and swaged a lip on the hole.

dwilliams is the only one who got it and offered a reasonable answer.
dwilliams wrote:
Tue Feb 13, 2018 8:09 pm
RT Machine wrote:
Tue Feb 13, 2018 6:39 am
What is the correct clearance between the shaft extension below the distributor gear and block on BB and SB fords ?
I never came across any official figures, so I used .0005 to .0015" and made some gauges on the lathe out of some round bar stock. I found a couple of blocks in the .005" range; I reamed those and drove bushings in.
As I understand it the hole in the block does not support the distributor shaft and it is supposed to be a clearance condition, is this correct ?
The hole in the block is what regulates the distance between the distributor gear and the cam gear. The bushing in the distributor is waaay up there. (and some of those are loosey-goosey, too, if you ever run into weird ignition troubles)

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