Honda-SOHC

SOHC.co.uk Forums => Project Board => Topic started by: Brett72 on November 22, 2020, 08:17:04 PM

Title: CB750 K5/6/7
Post by: Brett72 on November 22, 2020, 08:17:04 PM
Hi, thought i had better move to a proper page and away from the newbies intro page and start a thread purely for my resto. Its a big task i have taken on especially knowing that there is lots of parts missing and working out whats not there!!
Still lots of fun tinkering in the garage and playing on the net searching for bits. Not 100% sure wether or not to try and go as near to original as possible or because the engine is one year different from the frame go for a cafe/unique look and just make something i like.
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 22, 2020, 08:52:29 PM
just some of the photos as i go. I have a k6 engine but the carbs are from a K7. will they fit and work?
Title: Re: CB750 K5/6/7
Post by: Trigger on November 22, 2020, 09:33:13 PM
Look for a K1 to K6 set of carbs. They are better carbs and think the K7, K8, F2 PD carbs are only a match for the heads with the larger valves or, i could be wrong. Nope, thinking about it the early carbs are much more user friendly  ;)
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 22, 2020, 09:43:25 PM
Thanks Trigger. Might see if anyone has a set to swap/px
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 23, 2020, 10:04:50 PM
Evening, i have just taken the pistons off and have noticed that on numbers 1 and 4 the rings are fully compressed. Any tips how to get them off? They are soaking at the moment.
Title: Re: CB750 K5/6/7
Post by: Nurse Julie on November 23, 2020, 10:08:58 PM
Evening, i have just taken the pistons off and have noticed that on numbers 1 and 4 the rings are fully compressed. Any tips how to get them off? They are soaking at the moment.
Having seen him indoors, Trigger, come across this problem on customers engines quite regularly, I think the answer will be buy new pistons. They are usually compressed due to corrosion and therefore are of no further use to man or beast, or engine.
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 23, 2020, 10:18:53 PM
 :'(Thanks for the good news Julie. If i am lucky they will release in the potion i am soaking them in lol
Title: Re: CB750 K5/6/7
Post by: Nurse Julie on November 23, 2020, 10:25:18 PM
:'(Thanks for the good news Julie. If i am lucky they will release in the potion i am soaking them in lol
I'm sure they will release Brett, it just depends on how much piston material the rings bring with them 😢😢😢
Title: Re: CB750 K5/6/7
Post by: Trigger on November 24, 2020, 08:42:13 AM
:'(Thanks for the good news Julie. If i am lucky they will release in the potion i am soaking them in lol

From your picture, your pistons are part the point of no return and the bores will also be rusty and need reboring  :o
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 24, 2020, 08:26:25 PM
Thanks Trigger. The potion brought all the pistons up a treat and they were in great condition apart from one ring that i could not get off and slightly damaged the piston :'(. I am having trouble deciding how far to go with the engine. So far the top end is in bits and will get a full service, valve seats and pistons, bored or honed. The bottom end feels really smooth and there isn't any noticeable horrors that i can see. What are peoples thoughts? Should i continue to strip it all the way down or just rebuild the top and try and get a running machine? Open to suggestions.
Title: Re: CB750 K5/6/7
Post by: Laverda Dave on November 25, 2020, 09:56:29 AM
If the engine is already out of the frame and the head is off I would open it up to check everything is ok and also to renew the chains. If the pistons are rusty chances are something else will be as well and you don't want to be pulling that lump out twice :(
Title: Re: CB750 K5/6/7
Post by: Rozabikes Tim on November 25, 2020, 11:38:27 AM
If the engine is already out of the frame and the head is off I would open it up to check everything is ok and also to renew the chains. If the pistons are rusty chances are something else will be as well and you don't want to be pulling that lump out twice :(
Double ditto ;)
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 25, 2020, 12:05:39 PM
Cheers gents. I think that is the way i am going to go, just dubious on how much money i will put into this project. Will just have to drip feed without the missus noticing lol. Any advice on the best place to get parts around the UK?
Title: Re: CB750 K5/6/7
Post by: Trigger on November 25, 2020, 01:19:04 PM
All depends on your budget.
First i would make sure that the head is K6 and not K7/K8 as, you bought the bike in bits.
Any short cuts on these old 750 engines can bite back big time  ;)
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 25, 2020, 01:47:17 PM
Trigger, are there any easy ways of telling the difference between k6 and k7 heads?
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 25, 2020, 08:58:17 PM
Evening allMind has been made up about the engine strip for me. Turned it over to look at the bottom and oil contaminated with water poured out.!! Now struggling to remove the alternator flywheel centre bolt loose. Has Anyone struggled to remove this before? If so how did you get it out without doing any damage. Thanks
Title: Re: CB750 K5/6/7
Post by: Bryanj on November 25, 2020, 10:04:41 PM
Electric impact gun is easiest.
Title: Re: CB750 K5/6/7
Post by: Trigger on November 26, 2020, 07:59:30 AM
Trigger, are there any easy ways of telling the difference between k6 and k7 heads?

Bigger valves on the later head.

If you have water in the bottom end, then you have a rust problem and rust & corrosion is not your friend when assessing a engine for rebuild.
If you wish to lock the engine up, you can place a bar through all the conrods small ends.
Title: Re: CB750 K5/6/7
Post by: Brett72 on November 28, 2020, 07:30:17 PM
Evening all. i am after some advice if possible from the font of knowledge on here.  I am struggling to separate my engine cases. I have followed the good ole Haynes manual to the letter but i just cannot seem to separate them. Have tried lots of different size persuasion tools but they just wont budge. Is there a bolt that the book doesn't show? Do you need to remove the alternator off the shaft? i have seen many videos where they don't. Is there a bolt under the sump plate near the oil pump? Any ideas are welcome. Thanks
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