I have just spotted this both in the generic (1988) POH for the aircraft, and in the POH which actually came with it (the page also dated 1988):
This was actually done by one US based owner whose FSDO told him that he can install any engine that shares a type certificate with the IO540-C4D5D! This advice was obviously incorrect but clearly he cannot be prosecuted for following it. He went “offline” in the Socata forum when he was told of the situation 
But he did prove that the reason for the single shaft dual mag engine was not a lack of space behind the engine, which is the reason usually given. It was probably the ~$1000 cost saving.
However, the 2001 TB20 TCDS shows only the -C4D5D engine.
Could the earlier data be used to apply for a modification?
I don’t follow you on this : Both the D5D and B5D have the "D " suffix which designates “Dual” magneto, not 2 individual which is standard.
Hmmm… what is the difference in that case?
The IO-540 C4 D5D and -B5D have the same specified weight (410lbs) so they can’t be really much different. The only difference I could spot in the Lycoming Operator’s Manual is this one.
I thought all the -C engines had that. Reportedly it is a limit imposed on the prop end of the crankshaft…
The text next to the arrow pointing to the yellow area says exactly the same: the C4D5 is not affected by this restriction.
Hmmm, yes, well spotted. Funny that… this was discussed in the TB user group many times and nobody spotted that!
IO-540-C4B5 has different counterweights for use with Hartzell “compact” propeller.
Someone in the US has done this, documented it, and kindly gave me permission to post his writeups:
Converting to a dual magneto engine
He used a DER to do all the paperwork. This is somewhat similar to the EASA Part 21 route except that a Part 21 has no authority to actually do it, whereas an FAA DER can do it entirely himself. The $1200 DER fee makes Europe-based FAA DER jobs look like a complete joke (I was once quoted 2k for a DER package to put four holes in the floor of the luggage compartment to mount the TAS605).
The document is interesting reading anyway, not least for the discussion of electronic ignition options.
Peter,
is the actual pdf he mentions in his forum post accessible? Your link gives a PDF with the forum posts or am I doing something wrong? Would be interesting to read definitly.
