transimission

I feel silly asking this question, but where do you check/change the transmission oil on the M25 engine? I have an Ericson 27, so it's a bit difficult to access as is. I found a likely-looking bolt on top, but didn't want to go tearing into things without knowing. I have the short technical...

Just wondering if anyone knows if we have an HBW100 or HBW150 transmission and what the ratio is. My boat is a 1986 38-200 with an M40 based on a kubota V1502 block. I can't see the tag properly with the engine installed....maybe with a small mirror if no one knows. thanks shaun