Replacing dinghy valves

[My Wind Lass]

I have an older AB vinyl dinghy and one of the valves leaks slowly. I have a replacement valve but I cannot stretch the material around the valve hole enough to pull the old backing plate out and replace it with the new one. I attempted to replace it by screwing the external facing valve piece into the old internal backing plate. The problem is that even though the threads are the same, there is one more row of thread on the new valve so when I screw the new outer valve piece in, the gap between the two sides is too large to seal the valve.
I could attempt to create some rubber gasket to make up for the increased gap, but it seems to me that if the replacement valves come with an internal backing and the external valve, there must be a way to get the backing plate through the hole for the valve. I don't want to cut the whole larger or make any slits because there do not appear to be many adhesives that could patch that well onto the old material. Does anyone know how these valves get changed out or can anyone offer a suggestion to fix this problem. Thank you.

[paulajayne]

Pics would help.
Can you hacksaw some of the thread off?

[My Wind Lass]

Cutting the threads on the backing plate won't help because the way the new valve is designed, the threads do not extend as close in to the inside as the older ones did. I will try and get some pictures. Thanks.

[roberttigar]

google replacing dinghy valve and you'll get good step-by-step instruction. I did it. You have to slit the tube 1/2" on each side to get the old part out. then you cut a "donut patch and use the appropriate glue (vinyl or hypalon) to glue the patch on -after you insert the new valve part inside the tube. Bob