Model B distributor on Model A

[mcgarrett]

Any problem running a stock Model B distributor on a stock Model A engine? I can't imagine any issues other that not having to adjust spark advance. Just wanted to be sure I'm not overlooking something.

[alexiskai]

I've been doing a lot of work on this question over the last year and the main issue is that the springs in your B distributor are almost certainly not going to give you the timing curve you want. Most of them are too stiff and won't let the centrifugal advance kick in until 1200 rpm or even higher, and the total advance at 2400 will be just a few degrees.

To do this successfully you must have a degree indicator or a digital timing light that can measure advance. Otherwise you'll never be able to find the right springs for the correct timing curve.

[alexiskai]

A further issue is that no one is making Model B condensers right now, so you either need to adapt an A&L burn-proof condenser or acquire quite a few vintage B condensers and stress-test them to make sure they'll work. In my testing, 90% of NORS B condensers failed when heated to engine-bay temperatures.

You could also theoretically cut open a B condenser shell and insert a modern .25 mfd capacitor. No reason it couldn't be done but I haven't heard of anyone taking the trouble to do it.

[mcgarrett]

alexiskai
Very helpful information and much appreciated. I have an opportunity to buy a used B distributor but don't want to go down that road if it's a dead end. I'll give your comments more consideration before I do anything. Thanks again.

[mcgarrett]

alexiskai
Also, could you help me understand better how the Model B distributor works on the Model B engine but doesn't provide the proper advance characteristics on the Model A? I just want to get the nuances clear in my mind. Obviously, I'm not experts here, and you've spent a fair amount of time studying the differences to discover the associated problems. Thank you.

[Charlie Stephens]

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I have been running one for years on my '31 RDPU. Elongate the hole at the end of the condenser and make a short wire to connect the condenser to the distributer. Of course buy the best Model A condenser you can find, probably the one from Bratton's.

The timing is different so you will need a Model B front cover or follow some of the directions in earlier posts. You can identify the Model B cover by the location of the timing pin at the top of an oval pad.

[alexiskai]

Originally Posted by mcgarrett

alexiskai
Also, could you help me understand better how the Model B distributor works on the Model B engine but doesn't provide the proper advance characteristics on the Model A? I just want to get the nuances clear in my mind. Obviously, I'm not experts here, and you've spent a fair amount of time studying the differences to discover the associated problems. Thank you.

The B distributor provides 14-15° of centrifugal advance. It is limited to this amount by the design of the slots on the flyweights. Flyweights have been found with elongated slots that provide more advance, but it's not clear who made them.

A Model A engine with a stock cylinder head (4.2:1) uses a factory setting of 0° initial timing (spark advance at idle) and will typically need about 30° of total advance timing at cruising speed. The Model B engine uses a 4.6:1 stock head and is designed to run at slightly higher cruising rpms. The Model B timing cover, as mentioned above, assists a mechanic in setting the B engine's initial timing to 19°, which, combined with the 14° of centrifugal advance, yields total timing of approximately 33°. This can be adjusted a few degrees in either direction using a set-screw on the B distributor plate. (The B distributor does not allow manual real-time adjustment of spark advance.)

The problem when adapting a B distributor to the A engine is two-fold. First, you need to somehow reach the proper total timing for the engine. If you have a stock head, you need the distributor to deliver about 30° advance at cruising speed. Because the B distributor is mechanically constrained to 14° centrifugal advance, you need to set your initial timing, not at top dead center, but at roughly 15° before TDC. This is a difficult thing to do accurately without some kind of positioning device such as the B timing cover. Your starter then needs to be in sufficiently good condition to start the engine in cold conditions with the timing already at 15°. Some engines have trouble with this, and it could result in the loss of that characteristic low Model A idle sound.

Your second problem is that you need the centrifugal advance to begin advancing the timing within a certain rpm range and then get all the way to its maximum advance within a second rpm range. This is the timing curve. On a Model A, you would typically want the centrifugal advance to kick in somewhere below 900 rpm and to be all in by 1800-2200 rpm. In order to do this, the springs that control the rotation of the flyweights have to be of exactly the right length and strength so that the flyweights rotate out the right amount at the right rpm.

In my experience, the springs that people are getting in their B distributors do not deliver the correct timing curve. I don't know why. I don't know if the alloy used in the OEM springs was weaker or what. But across the board I've found that the springs are wrong. They're too stiff. Consequently you get no advance early in the torque curve and it does not reach maximum advance until you hit an rpm way above the Model A's operating range.

In my incomplete experimentation, I've found that modern stainless steel springs measuring 0.5mm gauge, 5-6mm diameter, and 15mm length are likely to give a timing curve that isn't ideal but is adequate. I'm still working on final settings and expect to have results in the next 1-2 years. (This is not my top priority project.)

[alexiskai]

Another thing I forgot to mention, for those considering this mod who have not seen a B distributor in person, is that the B distributor does not accept the stock Model A armored ignition cable. The Model B dropped the armored cable concept and instead used a simple wire with flag terminals between the ignition and the distributor.

Your choices are to either swap out your armored cable (and the ignition key, if you have an original pop-out) for the Model B wire or to fabricate an adapter that provides a receiver for the armored cable. Such adapters have been made. They're a little ungainly, IMO, but they work.

[ModelA29]

With initial 15° BTDC timing you'll need to be careful of possible kickback if you hand crank your car to start it. With the normal A starting timing at 0 there is no way it will kickback. This is also why your starter and battery need to be up to snuff.

[mcgarrett]

Thanks for the excellent explanation - that definitely clears things up about the differences and things to consider. I knew about the different timing covers, but forgot to take that into consideration about timing settings and total advance. Appreciate all of the comments.

[CWPASADENA]

The Model B distributor curve is better than most currently available aftermarket mechanical advance distributors. You can add a timing advance scale and mark the crank with a TDC timing mark and use a timing light to set timing. Having 15 deg or so at idle is not a problem. I have several customers running Model B distributors with very good results.

If the Model B distributor is in good condition, it will work great.

My opinion,

Chris W.

[alexiskai]

Originally Posted by CWPASADENA

The Model B distributor curve is better than most currently available aftermarket mechanical advance distributors. You can add a timing advance scale and mark the crank with a TDC timing mark and use a timing light to set timing. Having 15 deg or so at idle is not a problem. I have several customers running Model B distributors with very good results.

If the Model B distributor is in good condition, it will work great.

My opinion,

Chris W.

It's specifically good if you want to run a high-compression head in the 6.5 and above range, because you only need to hit 23° or 25° or whatever total timing.

It is a bit of a finicky design – prone to lubrication failure and more wobbly at the cam than the A distributor – but if you can get a good one and find the right springs, it does work.

[Ruth]

Originally Posted by alexiskai

It's specifically good if you want to run a high-compression head in the 6.5 and above range, because you only need to hit 23° or 25° or whatever total timing.

It is a bit of a finicky design – prone to lubrication failure and more wobbly at the cam than the A distributor – but if you can get a good one and find the right springs, it does work.

I'm curious why a B distributor is "more wobbly at the cam" than an A distributor. You would think if the bushing were good there would be no difference?

[alexiskai]

Originally Posted by Ruth

I'm curious why a B distributor is "more wobbly at the cam" than an A distributor. You would think if the bushing were good there would be no difference?

The B works a little differently in that the shaft comes up from below, but then the diameter narrows and it stops underneath the plate. The centrifugal governor drops over the top of the shaft and rotates on that smaller section. Then the cam is mounted on top of the governor, not on the shaft itself. As the ID of the governor wears over time, slop is introduced, and this can cause the cam to have irregular points clearance during operation.

[nkaminar]

The required advance curve is determined by a number of factors, such as what head you are using, what your mixture ratio is, and whether the engine is under heavy load or not. VW had a combination vacuum and centrifugal advance that addressed both rpm and the load on the engine. Once you determine what advance curve your engine needs then you need to set up the distributor to provide that curve. For my money, I will stick with the manually adjusted timing. It is what I am used to and it allows me to adjust the timing to suit.

[Joe K]

Alexiskai makes some interesting comments about "advance" between the Model A and the Model B.

Recommended one also read Vince Falter's page on similar beginning at below

https://www.fordgarage.com/pages/timingiseverything.htm
https://www.fordgarage.com/pages/bdi...toradvance.htm
https://www.fordgarage.com/pages/ignitorB.htm
https://www.fordgarage.com/pages/timingcovers.htm

All of these and more found far down on his lead page at https://www.fordgarage.com/

I've always had good respect for Vince. I'm told he has his "opinions" - but generally I've found him right in nearly every case.

You read - you decide for yourself.

I got into the differences between Model A ignition with manual advance and the Model B centrifugal advance. My first Gordon-Smith compressor came with a Model B engine and Model B distributor which seems to make sense for an automatically actuated function such as the kick-down/up of an air compressor.

My second Gordon-Smith Compressor will be based on a Model A engine core (available) but will have a Model B distributor AND timing cover. The B timing covers are available on Ebay for almost exactly the same money as a Model A cover - I suspect that many if not most sellers don't know the difference. Or maybe the pricing reflects actual demand?

Or you can find the "universal cover" - which is how Ford made them for a while - a cover which can be pin drilled at EITHER position.

Of course a Model A Air Compressor probably doesn't have the "operator critique opportunity" that a driving car would have. If it runs and compresses air, most would be satisfied.

You too could be the first on the block with a "trans" Model A motor. (hee, hee)

Joe K

[Synchro909]

I run a 6:1 head on one of my engines and for a somewhat original look, I wanted to try a B distributor. I had three of them and NONE worked worth a damn so I took one (the best) off to an ignition specialist. He tried but decided it wasn't reasonable (not impossible) to use it so he made up a modern (from a Nissan) distributor with mechanical and vacuum advance features. The result is magic. I have my 22° advance reliably and a great advance curve. He knew what he was doing.
My opinion of B distributors is VERY low. In fact, I think the reputation for B blocks cracking is due to those crappy distributors. I have never seen a new one but after a few years (and a few thousand miles), they don't advance properly. That leads to the motor running retarded much of the time and as we know, a retarded engine runs hot and a hot engine is prone to cracking.
IMO, there are decent distributors available that look just like the A distributor and have a good advance mechanism built in. I think that to use a questionable B distributor is tempting fate and it will come back to bight you on the @ss. I recommend buying a new one with advance already tested and proved. Trying to graft in a B dizzy might sound like an interesting challenge but I fear if you do it, you'll have a bigger challenge in the future - a new engine.

[alexiskai]

I recently had the opportunity to review a set of six B distributors that a friend had acquired from a collection. None were in working order. All six shafts were either worn beyond repair or fused to the casting by corrosion.

To be considered for use, a B distributor should be rebuilt by someone like Dave Renner who has the experience and the correct jigs to recreate the shaft with the weight plate attached to it. Once you have a usable shaft, it's pretty straightforward to rebuild the rest of it using original parts, with the aforementioned exception of the condenser and springs, which will need to be acquired new, and of course new bushings. Renner publishes a diagram showing common errors in the rebuilding, such as failing to drill an oiling hole in the upper bushing.

It's notable to me that all the horror stories about B distributors that I've heard go something like "It went bad and I switched to another distributor." Nobody seems to be interested in diagnosing the problem. I suspect bad springs and shafts, failing condensers, and poor quality B cams are responsible for much of the reputation.

[Joe K]

Plus 1 on Dave Renner. It's where I ended up with the Model B distributor for the Gordon-Smith clone.

He provided a whole new "rotating part" (including springs.)

Joe K

[Richard Knight]

The B distributor can be made to work on an A just fine. You just need to know where you want your initial timing too be and where you want your total timing to end up. Lots of good info here. Part of making the B distrib work on an A is the springs that are being supplied. Alexikai has sourced appropriate springs. As has been said the centrifugal advance is around 15-16 degrees depending on wear on the weight pivots and the slots. Initial timing of around 8 to 10 and the lighter spring will bring the full advance in by 2k rpm. And you will need a timing light and nurex degree thingy. I hope this helps a little.