Hey Bingobelle,
Here is a link to the scematic of the BHP451 on the Canadian site. We have a guy who works here in sales who was an outside demo guy for Makita for several years and he's very familiar with what you're talking about. He said there was a rolling change in the gearbox and two change rings available. Take a look at the scematic here:
http://www.makita.ca/data/upload/tools/BHP451%20-%20SEP2008_parts_English.pdfNotice how there is a type 1 and type 2 change ring?
You'll have to find out what one fits your drill but I assure you, you can replace this one part.
The part numbers are as follows:
418763-8 SPEED CHANGE RING (TYPE1)
419558-2 SPEED CHANGE RING (TYPE2)
Hope this helps,
Terry
KMS Tools & Equipment Ltd