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From my experiance, the Ford synchro inner is the same as NP420 in our trucks. They will not work in the early transmission. The difference is hard to see in a picture but easy when in hand. If you look up the NSN of your synchros for the 420, it will cross reference to the Ford part.

The synchro used in the early M37 trans is similar, yet VERY different. The early unit sliding clutch has a groove the fork fits into when assembled. The 420 synchro clutch fork engagement area is much thinner, and as already discussed the fork has the groove in conjunction with the nylon inserts. The early unit is actually pretty simple to rebuild worn areas if you have machining capabilities, the 420 synchro/fork arrangement is much more complicated to rebuild (although it is possible) because of the design. The 420 as a unit has far heavier shafts, etc., internally as compared to the early trans used prior to 1958, so it is the M37 trans of choice. As a unit the 420 will interchange with the early unit, problem is 420's are a lot harder to come by, early units are plentiful.

I'll mention this also, the nylon fork inserts are not part of the 435 small parts kit. The kit has many needed parts to accomplish a successful build, snap rings, etc., but inserts are not included in those kits, (never understood why as they are certainly a high maintenance item) so don't let someone sell you a kit thinking these inserts will be included.

MSeriesRebuild wrote:I'll mention this also, the nylon fork inserts are not part of the 435 small parts kit. The kit has many needed parts to accomplish a successful build, snap rings, etc., but inserts are not included in those kits, (never understood why as they are certainly a high maintenance item) so don't let someone sell you a kit thinking these inserts will be included.

Novak sells two NP435 small parts kits: one for the guts of the trans, and one for the top cover. The nylon inserts are included in their top cover kit, or you can buy them separately.

One other note about ordering NP435 small parts kits: do not expect to use everything in the kit in an NP420 rebuild. You will have left over parts.