Gearbox issues

[les]

Last year I replaced my van gearbox because fourth gear would occasionally not select and it really was only now and then. However I had another box, so I changed it. I’m thinking in the warmer weather I might take a look at the original. So basically I’m looking for any clues on what the problem could be ?

[Bill_qaz]

3rd /4th sliding ring or 4th gear synchro best guess

[les]

Thanks Bill, that’s noted !