Honda CB750 Sandcast

The Restoration Shop => Frame and Chassis Related Elements => Topic started by: Garry32 on October 15, 2015, 01:33:38 pm

Title: Chain guard changes
Post by: Garry32 on October 15, 2015, 01:33:38 pm
Can anyone assist with information on the following :
The early chain guards were short with nothing on the interior for support.  Did they then go to the long one with the inner brace and if so what number did this change start showing up?
Thank you Garry :)
Title: Re: Chain guard changes
Post by: Steve Swan on October 15, 2015, 03:40:59 pm
No long guards on sandcast.
Title: Re: Chain guard changes
Post by: Garry32 on October 16, 2015, 12:39:08 am
Thank you Steve for the reply, I New all Sandcast where short but was just curious when Honda changed to the guard with the bracket on the underside of the guard to give it strength ?
Title: Re: Chain guard changes
Post by: Steve Swan on October 16, 2015, 02:21:03 am
Honda Service Bulletin #22, 8/28/70, "modified" chain cover 55 mm longer "installed free of charge on any CB750 manufactured prior to Frame Number CB750-1021880."  There is no mention of any steel reinforcing bracket to go inside the guard.  i'm pretty sure the "modified" guard was same as the short guard, i.e., same swing arm mounting from sandcast #1 up when diecast 21880 was fitted with longer guard.

Honda Parts Bulletin #750-35, dated August 28, 1970, "the following new parts  are applied form Frame Number:  CB750-1026845."  The parts are pictured in the parts bulletin to include the bracket you refer to, pn 40512-300-000.  Honda also call for the side plate on rear sprocket with the idea it will help keep the chain form derailing too far, should chain come off rear sprocket., pn 41211-300-000.   Did the swing arm also change to accommodate fitting this metal bracket inside the guard and attaching it to the swing arm ?  

Comical in a strange sort of way, at least from an attempt to redesign parts to prevent a loose chain from derailing and/or wadding into the rear crankcase wall, PB #750-36 (revised) calls for pn 11353-300-010, case protector and pn 11354-300-000, rubber chain guide.  both these parts fit inside the redesigned 11351-300-040, cover, l. chain case.  Honda by that time, was having to deal with by then a fairly significant number of warranty claims for broken crankcases.  Factory was providing unstamped sandcast replacement cases to satisfy warranty claims.  all because chain technology of the day could not manage the horsepower of an engine producing 68 hp, resulting in excessive and unexpected stretching of chains greater than recommended service intervals.  and today we have stock out of the crate 180+ hp chain driven bikes and chain failure is generally not thought of.  
Title: Re: Chain guard changes
Post by: Garry32 on October 16, 2015, 03:28:02 am
Thank you Steve, that answers my question,with more info to keep on file.
Thanks again Garry :) :)
Title: Re: Chain guard changes
Post by: hondasan on October 16, 2015, 06:33:47 am
The swing arm change at 1026845 was for the substantial rear chain guard mount to become a much simpler tab with captive nut, thereby enabling the steel re-inforcing bracket to be used inside the longer plastic chain guard, the chainguard itself then being attached to the swing arm using a much shorter mounting bolt (M6 x 16 from memory). Previously the short chainguard (and long guards between 1021880 and 1026845) were rear mounted using a longer (M6 x 71mm bolt), preventing chainguard removal without first removing the left side exhausts (or at least loosening the silencer mounting bolt).


Chris R.
Title: Re: Chain guard changes
Post by: Steve Swan on October 16, 2015, 08:43:05 am
Nice to see you again, Chris !
Title: Re: Chain guard changes
Post by: Sgt.Pinback on October 16, 2015, 10:27:33 am
I am actually Selling a K0 chain guard on eBay. Far from being good enough for you, but at least no repro.

Item Nummer

201449581278

Please delete post if not allowed.