Welcome to our resource for MG Car Information.
MG MGA - Distributor question
The attached (I hope) photo shows the moving contact breaker plate and the base plate from my distributor. How far should the points clamping screw go through the moving plate? Should it be clear of the base plate?
| Should it be clear of the base plate?|
Absolutely unless this caution moving plate might no longer be movable. Easy to check after with a strob. lamp
with the proper advance curve
|Thanks, that's what I thought. So this screw should be very short then. I wonder why this long one was used when the distributor was refurbished? I've already shortened it by at least an eighth of an inch.
|Does your distributor still use a vaccum advance?|
If not the plate does not move anyway.
The centrifugal advance is under the plate it advances the point cam in relatiion to this plate.
The vac advance moves this plate in relation to the cam.
|R J Brown|
Yes it has a vacuum advance
|Phil. With the points installed, the screw should extend no more than about .080" below the plate holding the points. This will allow full movement between the two plates, including the disassembly of the two for lubrication while the points and condenser are still attached to points plate.|
| Hello Phil Parmenter,|
Once upon the time, tuning my dizzy, I lost the original points screw. Found a correctly threaded one, not that easy in continental europe, fortunatly in a spare box but too long. I am not sure, but that might be an explanation. Surprisingly did the car take all its advance curve?
|Hi, I'm grateful for your comments about this and I'm really sorry to keep on about it. In the attached photo you can see the base plate looking from the bottom up. At about the middle of the photo you can see a hole in the base plate which corresponds with the threaded hole in the moving breaker plate for the points securing screw. Why would this hole be there? Do you think that the base plate has been drilled and a long screw used for the points in order that the moving breaker plate can't move? If so any idea why?|
What distributor are you using? It is not the baseplate of the MGA DM2. This is the picture of mine.
|I may have this all wrong (whats new!) but on all the several 23D and 25D distributors I've played with the points-clamping screw goes a good way through the base plate, sticking out about 0.22". Then the two plates are limited in the maximum arc of rotation and it's only possible, I thought intentionally by design, to disassemble the two plates by taking the screw out. The screw is a simple flat top, slotted one with shakeproof washer, about 0.44" total length.|
Have I got it wrong? There are several on my bench like this!
|The markings on the body are -|
DM2 (this is a bit indistinct but I'm fairly sure)
|There were changes over time in the plates, maybe with different part numbers, maybe with the mysterious letter suffixes. (Letter suffixes mean the part is different, but basically interchangeable, but may require other matching mating parts. In this case it would be the screw.)|
I've seen early solid bottom plates, which must have short screws.
Phil's distributor could take a long screw, but it would restrict the vac advance travel; it probably came with a short screw.
Later dizzys or replacement plates are slotted so that you get full travel with long screws. Bruce is correct that you have to take the screw out to separate the plates, which I always thought was an excellent little engineering touch.
There may also be, or have been, different plates supplied OE for vac & no vac dist, but frequently the replacement parts are "rationalised", always to the latest or most universal design.
I remember being really happy when the long screw plates came out, since those little short screws are a PITA to do if you have to change points in the car beside the road, and customers having no sympathy, it was always in the cold or rain!
Bottom line: It's up to the mechanic to fit the right parts, or modify what's there to work correctly. Not hard to drill a hole in the plate so you could use a long screw.
Most interesting. Same as mine. Both made in July 1957, but different base plates. Perhaps someone else can confirm which base plate is correct.
On mine, the set screw has to prortude through the lower plate to act as a stop to prevent over-rotation of the top plate (vacuum advance movement) into the lock mode (upper slot in my photo).
|Thanks guys, yes it is interesting. As I've said mine has a long screw so with this base plate it can't turn at all. If I use a small screw then there would be nothing to stop the top plate from over rotating. Mind you although you say yours is the same model who knows what parts have been changed over the years. Mine was refurbished by Holdens in 2005. I might contact them and see what they say - I thought they were supposed to be a good company as far as distributors go.|
|Mine was refurbished by Aldon in 2007. No comment!|
|Steve and Phil. The vacuum advance can limits the movement of the points plate in relationship to its base plate. The system, if working properly, should not be able to over rotate the points plate and allow it to separate from the base plate. Hence, no long screw necessary in normal operation. |
|The screw does keep the top plate from rotating too far and coming off the lower plate, but this is solely for the benefit of ham handed fiddlers. It has no functional use, since the vac advance also keeps it from rotating too far. The upper slot with keyhole in Steve's pic is not "lock mode", but rather "disassembly mode", so you can separate the plates. The plates are wearing & replaceable service parts, and at some point all replacements went to the later slotted type as shown in Steve's pic. As such, any type is correct if it has the right screw fitted. For Phil, if the center spigot is not worn and sloppy, I suggest slotting the clearance hole out so you can use the long screw - it really is a lot easier to deal with. You can check the requisite slot length by sucking on the vac capsule with a pump. I used to do this when the short screws were replaced with the long ones, even if I had short screws on hand.|
|Brilliant, thanks very much everybody. I'll do what you suggest FRM and file away the base plate so the longer screw can rotate. As a matter of interest what have I been losing with the top plate not being able to rotate?|
|So, with University Motors out of business, where is a good place in the US to send an MGA distributor for rebuild? I've got one that was totally disassembled by a teenager many years ago, that I would like to send off to see if it can be salvaged.|
You are losing the vac advance, which translates to part throttle fuel economy = cruising mostly. Some later cars (emissions) also lose low end response.
Put the plate back in the dizzy and connect the advance spring link, and screw the manual vernier setting down tight - this is one extreme of travel. Then apply vacuum to the vac advance unit until it doesn't move any further and unscrew the manual setting to its limit, that's the other extreme. Total req slot will be less than the width of the slot in Steve's pic, because some units have more advance, plus some for unit to unit variance.
I thought John at University Motors was still doing bench work. You might want to give him a call.
I recommend Advance Distributors, LLC
1149 Quincy St
Shakopee, MN 55379
This thread was discussed between 16/02/2010 and 24/02/2010
MG MGA index
This thread is from the archive. The Live MG MGA BBS is active now.