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MG MGB Technical - 25D Distributor ID

I bought an A to restore that came in pieces. The distributor that came with it is not correct. It is a 25D with 41288 B stamped on the outside and the numbers 30 and 69 stamped below. Am I correct in assuming this is for a B and, if so, can someone please tell me which model years it will fit?

Thanks.
R Phillips

41288 was used on non-North American 18V 581/582/583 high compression engines from Aug 71 to Nov 73.
Paul Hunt 2

R-
Here's a bit more info in the 41288 that you might find useful-
Vacuum Advance Begins @ 4” Hg
Vacuum Advance Ends @ 12” Hg
Maximum Advance in Crankshaft Degrees: 16°
Steve S.

The notation is very interesting; what is the correlation between crankshaft angle and the last two digits?

My '67 MkI BGT has an 45D distributor in an 18GB engine. I'm wondering if this degrades the performance of the car as I understand the car would have been factory fitted with a 25D type?

Thanks,
Brian
Brian McIlvenna

Brian-
It depends upon which version of the 45D4 you have. Look on the distributor for a five digit number that starts with 41 and tell us what it is. I can probably provide you with its specifications.
Steve S.

I'm not aware there *is* an correlation between the distributor reference number (41nnn) and crankshaft angles, the reference numbers just seem to move upwards over time, i.e. they just used the next available number when they needed a new curve.

An 18GB in a 67 would indeed have used a 25D - a 25D4 40897 for High Compression or 40916 for Low Compression.

If you look at the body of your distributor for the 41nnn number (it is usually underneath when installed) and have a look at http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Distributor Curves' if it was an MGB distributor it will be listed, with its spec, and you can compare it to the 40897 (for example). The UK 25D curves are very different to the UK 45D curves, for example the 40897 maxed out at 20 additional degrees at 2200 rpm, whereas the chrome bumper 45D for example maxed out at 30 degrees at 6000 rpm.

However the original factory curves are pretty irrelevant these days, with the very different fuel we have. Unless you are going to go for a long-winded and expensive excercise to determine the ideal curve for your engine, then spend more time and money finding springs and tweaking them to suit, the best you can do is set the advance just short of pinking at any combination of throttle, revs and load.
Paul Hunt 2

Thanks for the replies. With any given distributor, can the advance point be adjusted, short of changing the unit's internals.

I'll check my distributor tomorrow.

Thanks again,
Brian
Brian McIlvenna

Not quite sure what you mean. The timing can be advanced or retarded an equal amount throughout the rev range by twisting the distributor relative to the engine, as both centrifugal and vacuum advance are expressed as being *additional* to the static advance. Twisting the distributor changes the static advance, so changing that from, say, 8 degrees to 10 degrees BTDC, will add 2 degrees to the total advance throughout the rev and throttle range. But if you want to alter the amount of additional advance at one point in the rev or throttle range compared to another, then you have to change the springs (centrifugal i.e. rev advance) or vacuum capsule (throttle advance).
Paul Hunt 2

R Phillips, the 41288 is failry close to your original A distributor in advacne curve and will be more cinsistent than the pre-'61 DM2 distributors.

30 69 means it was built in the 30th week of 1969. 41288 has no reference to any specs other than the application of MGB 1800. They 41288 WAS used in the US as well - from 1968-1970. The vacuum unit should be a 5-13-10 for a US spec distributor.

Brian, there's no loss of performance when switching to a 45D distributor from a 25D, unless the ignition curve is "off". Yours is probably an aftermarket unit, as opposed to one borrowed from another car. In that case, performance should be good.
Jeff Schlemmer

This thread was discussed between 31/12/2007 and 06/01/2008

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