Yes the advances are different on some. Most of the letter series came with a 40 degree advance.
Lots of later tractors came with less advance. Also a lot of letter series distibutors got changed from 40 to 22 degrees when fire crater piston kits were installed.
Most of the distributors have a symbol letter that indicates what advance originaly came in them. Also a date code and serial number. That info is stamped into the round base where the two retaining clamps go. Usually unable to see the info unless the distributor is removed from the drive housing.
Most units with different advance or total rates have distributor shafts and or advance springs different.
Just today I checked out a distributor from a 350 gas. When apart I found rusted advance parts and a shaft with 16 degrees total advance. Should have had a 22 degree advance. Someone put a LP gas on it.
When I said letter series I was leaving the supers out. Lots of them were 30 degree total or less than 40.
Like Bob M said the diesels turned the opisite dirrection and only used 0 to 8 degree advance units.
If you like to tinker about any distributor can have the total advance changed by changing the hole size in the distributor shaft. Helps to have a distributor machine but with the right kind of timming light can be done on a tractor by trial and error. Easy if making the hole bigger for more advance and a little more work making them less.
This post was edited by D Slater at 09:13:37 11/05/12.
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