Ok, I have taken a few dimensions and have the following facts, not factoids...
A standard Manitou 1 with steel stanchions has 170mm of engagement (the length of stanchion inserted into the leg)
A Manitou 4 with aluminium stanchions fitted with Englund Air Carts has 160mm of engagement.
these modded M1's have 120mm of engagement.
A standard M1 has a axle to crown length of 400mm, compared to 430mm for these modified ones.
So, maybe not an ideal amount of engagement to be honest, but I weigh almost 16st :shock: and havent managed to break them. Admittedly I have not used them very much, but I have bounced up and down on them quite a lot to check travel etc.
I could make them a bit safer by removing the 15mm spacer, shortening the air cartridge mounts by 15mm and then fitting stops to limit the travel to ~50mm. That should give 150mm of engagement and return the a-c length to 400mm.