Question 1

What is the weight capacity of these plugs ?

Accepted Answer

I used these plugs on a mirror weighing in excess of 20kilos, 2 plugs at 400mm apart, but the wall was really solid though.they are also holding up kitchen cabinets. In a solid wall it's a great anchor. 
Note: try not to use hammer drilling to do thé holes for the plugs. If you get a drill bit that can do masonry holes without the hammer action it holes way better, if not, use a drill bit 1mm smaller. 
Once that screw goes in, it takes a lot to come out.

Question 2

are these suitable for plasterboard cavity wall

Accepted Answer

Depends on what you need to mount on the wall and the space between the plasterboard and the wall/ studwork the plasterboard is affixed to. I use these for fairly heavy items (large mirrors) on my wall that consists of plasterboard fixed with dot and dab to the wall behind, So with a 15mm board and a 15 to 20mm gap to the wall these are ideal. For lightweight items with a hollow behind the plasterboard use the shorter Duopower. For heavy items on plasterboard with a hollow behind I use the two part metal cavity fixings that use have m4 or m5 threads.

Question 3

What would be the best screws to use ? ty

Accepted Answer

It doesn't matter too much. Any regular zinc plated single thread screw will work, including wood screws, self tapping screws or coach screws. It's the size you need to pay attention to, I believe you need either 4mm or 5mm screws. The length would be the "plug length + fixture thickness + 1x the screw diameter", ie 55mm + fixture thickness.

Question 4

Are these the right size for 6mm x 50mm coach screws? It's confusing - not sure if you match the size to size of screws or if it is different.

Accepted Answer

The 6mm x 50mm refers to the drill hole size. 
I would recommend a 8mm drill hole for a 6mm screw. Fischer do a 8mm x 50/60mm duopwer plug.
Hope this helps.

ASIN	B01LXRVBGC
Assembled Diameter	6 Millimetres
Batteries Required?	No
Batteries included?	No
Best Sellers Rank	300,542 in DIY & Tools ( See Top 100 in DIY & Tools ) 1,139 in Plugs (DIY & Tools)
Colour	Red/Gray
Customer Reviews	4.8 4.8 out of 5 stars (1,099)
Cutting Diameter	6 Millimetres
Date First Available	7 Nov. 2016
Finish	Two Tone
Guaranteed software updates until	unknown
Head Style	Flat
Included Components	100 Plugs
Item Package Quantity	1
Item Weight	140 g
Item model number	538240
Manufacturer	fischer
Material	Nylon
Measurement System	Metric
Number of Pieces	1
Part Number	538240
Power Source	Hand-operated
Shape	cylindrical
Size	Ohne Schraube
Style	Single
Tool tip description	Universal

Fischer 538240 - Duopower 6 x 50, Red/Gray

Details

Details

Description

Specifications

Common Questions