I agree with the Sikaflex to seal the seams, or some Epoxy putty.

A friend of me did make holes of 6 mm at the steel plate at the edges rails but also at the tunnel rails, he did fill weld the holes and he is pretty shore that it is better than stitched welds. No distortion of chassis, like some people think about.
The fill welds where made jumping from corner to corner to spread the heat, and the plate is about 6 mm from the edge so that the holes are close to the inside wall of the tube.

Regards,
Cobra289

Somebody correct me if wrong and please do,is there not an ever present possible problem with the Galvanic reaction between ally and steel to consider,

The old white powder problem (not drugs Gents ) seen in landrovers etc.

Just re-reading the post's and Adrian-H's idea of spot welding the steel sounds good too.
And yes you need a spot welder.
With regard to bonding I have seen a vehicle roof glued on and then picked up with a crane mounted magnet and shook about violently and it held,I myself have used similar stuff to replace panels but it is not cheap.
Your thoughts ?

On my floor i put a thin weld every 3" starting from the side rail and letting it run down and bond into the floor sheet, 16 gauge steel. After powder coating the finish has bonded the rest together very strongly. Its had my 16st clambering all over it recently and its stood up fine.

It may not have been the best way but it seems more than adequate.

Adrian
It is possible to spot weld one side ie: single sided weld and the only difference is that a large earth is used as opposed to the clamp method you referred to previously,Basically all you do is clamp a large copper earth tag to somewhere nice and clean and place the other electrode on where you want the weld and pull the trigger. The one we use at full duty cycle will blow holes in three body panel skins no problem and when you pull the trigger the cables lift a few inches. Well known for blanking SIM cards and screwing up mobiles.

Cheers ALL

what about a sheet of carbonfiber riveted in place?? this is what i was thinking about but i never thought about the rivets coroding and stuff.

would this be a good idea or am i been an idiot

Carbon fibers in contact with aluminum is a big problem. The aluminum will go into powder very fast. [Think and the rivets]
So use stainless steel rivets would be better. Check if they are water tight rivets, and don't forget that they are heavy to install. [you will need more force]

Regards,
Cobra289

You been "borrowing" manhole covers ...........:rolleyes:

I hope to plug weld the steel floor to the chassis. I will follow this up with spot welds on the inside of the chassis as per Spud69. Has anyone used this technique on their roadster. I've just had a practice at welding 16 gauge. Seems good. The problem is how do you seal the outside of the floor. Please see attachment.

Peter

This like perfect position.
Seal with the car industry kits, ask for the bond type that is used for car bonnets.
Other solution is to use Epoxy Kit.
For all the the aplications the key is cleanness, make de bond area as clean as possible.

Regards,
Cobra289

Seam welded mine all round the underside to stop any water getting in. Painted the inside which has filled up the gap between the floor and rails. Nothing fancy but looks effective at the moment.

Neil