CB750F1 v K series Crankcases

[timbo750f1]

Hi. I'm a newbie so hope I do this correctly! I have just purchased a 1977 CB750F1 as a restoration project. When trying to set the timing I noticed that one of the lugs for the points backplate screw was broken. It's integral with the lower crankcase. I could try to get a repair done but was thinking it might be cheaper to get a used lower crankcase. There are quite a few out there but they are K series or F2's. I was wondering what, if any, differences there are between the F1 lower crankcase and say a K6 or F2? I know the F1 was the first to have a rear disc brake and someone said this meant a slight change in the case, but I don't know. Anyone able to enlighten me?

[hairygit]

You cannot replace just the bottom half, they are very accurately machined as matched pairs. Trying to mix and match will lead to much expense, aggro and heartbreak. Either get yours repaired, or buy a replacement pair. Welcome by the way, and good choice of bike, the best looking of the 750's.

Sent from my X6pro using Tapatalk

[Trigger]

This is a cheap repair to do, you just have to find a very experienced alloy welder. I charge £25 to £50 for this type of weld, drill and tap. This will give you some idea as to cost.

[JamesH]

Hi Timbo. Trig and Hairy are spot on. Find a good welder to effect the repair and best of all it's unseen and MUCH easier than removing the engine, stripping and rebuilding...

[timbo750f1]

Thanks for all the great advice. Think the repair road sounds best, especially as it's unseen. One other route I had thought was electronic ignition which probably takes less 'abuse' - no contact type that is, and possibly sticking with just 2 lugs, but I'm a bit of a perfectionist and I know it's there even if no one else does. I coveted a CB750 F1 when I rode my CB400 4 in 1978, it's only taken me almost 40 years to get one! I do prefer the F1 to the F2. I do intend stripping the engine and rebuilding it - perhaps over the winter! Just getting it roadworthy at the moment - brakes were completely seized but I've managed to free them up and a re-build kit is arriving any day. Next is the carbs cleaning and synching! I'll try attaching a picture of the bike. It was last on the road in 2006 and has 31,000 miles on the clock which is genuine going by all mot's which came with the bike.