RD690S/R685S info

[tl1975]

Hello. I have a 1988 RD690S (EM6-250L 4VH and 5 speed) and wanted to ask you all a few questions about the interchangeability of parts from a late 70's/early 80's R685S (EM6-237 2VH and 5 speed). Both are cab & chassis now and I'd like to make one truck out of two, IF it's possible. The RD690S has a good frame but bad cab and engine, and the R685S has a good cab and engine but bad frame. Hoping some of you could help me out with the info. My questions are:

- Can the EM6-250L 4VH engine be directly swapped out with the EM6-237 2VH as far as engine mounts, space, etc? 
- Will both 5 speed transmissions be interchangeable using the same mounting?
- Will the RD690S cab directly swap out with the R685S cab? How about the hood?

Any info is much appreciated. Thanks!

[fjh]

11 hours ago, tl1975 said:

Hello. I have a 1988 RD690S (EM6-250L 4VH and 5 speed) and wanted to ask you all a few questions about the interchangeability of parts from a late 70's/early 80's R685S (EM6-237 2VH and 5 speed). Both are cab & chassis now and I'd like to make one truck out of two, IF it's possible. The RD690S has a good frame but bad cab and engine, and the R685S has a good cab and engine but bad frame. Hoping some of you could help me out with the info. My questions are:

- Can the EM6-250L 4VH engine be directly swapped out with the EM6-237 2VH as far as engine mounts, space, etc? 
- Will both 5 speed transmissions be interchangeable using the same mounting?
- Will the RD690S cab directly swap out with the R685S cab? How about the hood?

Any info is much appreciated. Thanks!

Front mount maybe the question here as , the engine evolves to 4 valve the mounts on the front start to change from trunnion mount to pedestal mounts !It will all depend on the donor engine front mount !Long as both engines have a front steel cover all should fit! as for cabs the rd has unique mounts how ever its all bolts and nuts so I believe this here is doable as well with some swapping ! not sure on the  hood you would have to compare them your self see if its doable! 

Edited May 29, 2022 by fjh

[youngstown.bulldog80]

I believe the guy above is correct.. you may have to fab up a mount for the front. I could be misinformed but I had always thought the RD had the beefed up frame but for the era of your trucks maybe a cab swap is possible.. do some investigating and take some measurements etc. As for the hood ?? And double check the 5 spd trans in each.. if they’re both the same model 5 spd such as TRL107 or TRL1076. There are a lot of knowledgeable people on this forum so I’m sure you will get all the information you need.