so i have read some things about  being metric or standard...can some one explain or link it for me?..I seem to also have a slide cover with a Weaver style rail  on top..


neither one of my SAR-48s work with it..it seem sto be a 1mm to wide?.,,,is thsi the metric/standard thing?..

If so ..what the hell..I thought Springfiled said there parts are all interchangeable?

Inch and metric covers will interchange. Some inch covers (like the scope mount covers) have a tab that fits into cuts made into the back of the receiver which metric receivers are missing.
Does the cover have any screws on the side? Are there any markings as to who made it. The cover could be out of spec. There are slight differances in the upper receivers but it is down to a few thousandths. Some covers will fit tighter than others but usually not that far off. Some of the ARMS covers were really thight on some receivers as were the williams hi and lo pro mounts.

You can find tons of info at: www.falfiles.com

sweet thanks chuck...just found falfiles..thank you

Metric is "standard" for the most part... the alternative is "inch" pattern, where design (not dimension) modifications were made to some parts.  In some cases the parts interchange, in other cases they don't.  

As Chuck1 said, it sounds like you might have a cover designed for scope mounting, and possibly an inch pattern one that had tabs at the back which require recesses in the receiver to match.  Pics would be helpful.