CB750 K3 Oil leak

[Bryanj]

With oil all over it does suggest that either the head or block is not flat or that they are not being clamped together enough by being held too far apart. I know that there are several different lengths of dowel pin, perhaps they have been mixed up? Try fitting the head to barrel with no gasket---it should allow the mating surfaces to touch ensuring enough squish of gasket and O ring

[sim2011]

Update
I've taken the head and barrells of to SEP to be checked out so I'll let you know what they find.

I managed to find the original Honda head gasket and its mighty different to what Honda supply today See pics , the 1973 one also had gasket sealant around the cam chain tunnel the 2010 one doesn't !! The 2010 one is also bereft of those copper sealing rings .
Also I have noticed that on the 73 version all other holes (not the copper Ones) have the same sealant applied as the can chain tunell
Any thoughts on this will be welcomed.

[K2-K6]

Hi again. To me that would confirm that you need a sealant at key points if not all (I've always covered total area) on any gaskets pattern or otherwise.

These are my reasons:- Honda clearly designed (original gasket proves it) to have some type of sealing additional to the gasket material around the oil ways.

I don't believe that sufficient torque can be exerted to do it totally dry with no additional sealant of some sort (copper rings, factory applied sticky-bits).

If you don't have that original spec of gasket OR something superior to it you'll have to use a substitute.

You can't increase the torque without stepping outside the original design spec, anybody that knows anything about the engineering design process will tell you that looking at this design (layed down in 1967 ish) was an absolute masterpiece then and still is an example of how to make a first off reliable engine from scratch; just one example is the stud design that is waisted (thinner) in the centre than at the threads to give equal torque load over the whole length and cross-section of each stud!!! also how the bottom threads are buried down into the casing so that the load is pulled from further within the material again to spread load indicates how thorough the design team went to work.

To me, the risk of using Blue Hylomar is only to avoid ANY loose sealant floating around as it survives in oil. The rest is all upside, as previously posted (by rigwit?) this stuff is seriously good.

With a nice new tube you can get a really thin spread coating onto the gasket, it even looks and feels like the original coating when it's dry.

Be interesting to see what your barrel / head inspection shows, let us know.

Maybe a bit too enthusiastic design appreciation there, maybe should be filed under "soapbox" but they are really good...............really.

[Bryanj]

I dont ever remember seeing a Honda gasket with copper rings in it

[Spitfire]

Never seen copper rings either, but have only worked on CB750's up to K2 and F1/F2.

Cheers

Den

[mick]

Just my 2 cents worth , i've never had to use sealant apart from under the cam tower pucks & the crankcase halves, just made sure all surfaces are nice and clean, cheers Mick.

[sim2011]

All
Thanks for the input , I have just been looking at the parts manual fiches and the K2 US head gasket clearly shows rings of some sort around the holes so these will be the copper ones in my pic , my guess is that they used this on mine as its a Dec1972 build date.

[Bryanj]

I have a parts book so early its on thick individual pages dated 1974 and yes it does look like there is something but there was/is only one part number for the gasket from the CB750 to the K4 (thats as far as the book goes)

[Bryanj]

Just been playing with DS website and a K2 top gasket kit is discontinued but the 750A top set exists at

[rigwit]

To me, the risk of using Blue Hylomar is only to avoid ANY loose sealant floating around as it survives in oil. The rest is all upside, as previously posted (by rigwit?) this stuff is seriously good.

blue hylomar is actually for pipe threads, but is a great gasket sealer for uneven parts it is non setting in a sense, but does harden up a bit -  like half way from blue tack to Plasticine. Golden hylomar is non setting but isn't as resilient as the blue for imperfection filling and red is totally setting only ever used red on waterpumps and diff casings. another tip is I always coat non metallic gaskets with a layer of grease both sides helps with instillation sealing and doesn't make the gasket dry out and go brittle so if ever have to take cover off its a doddle.............

[BigAl (Alan)]

Hi Simon,

I have a K4 (same colour)  not changed the top head set, but I did overhaul the carbs last year and had a few problems with  setting up (purchased a standard CB750 carb kit,120 main jet)
Anyhow after a bit of reseach I found that the USA immision laws changed in 1973 and the carb main jet was replaced with a 105 unit (K3,4,5) which runs hotter & leaner...I think..??.. Honda also modified the head gasket to compensate for the extra heat, hence the copper rings on the origonal gasket.

Hope the information helps, and please let us know if the new gasket has copper rings

Alan

[sim2011]

To all

Quick update, I've just collected the barrells & Head from SEP and they have skimmed both.
The head was warped and the barrels had a slight dip in the middle.
So now I have
New Honda gaskets
Flat metal surfaces.
I am going to use sealant just havent decided yet on Blue Hylomar or clear silicone.
I'll keep everyone posted .
The new Honda head gasket does not have copper rings
Thanks for everyones input.
Si

[Seamus]

How much out were the head and barrels? Please keep us posted with reference the rebuils and subsequent oiltight engine.

Good luck

Seamus

[K2-K6]

It'll be interesting to see how you get on, did they say how much they skimmed from each surface?

Hopefully you'll soon have it oil-tight and able to enjoy a good run out, thanks for keeping us informed it's alway's good to see the full story and eventual progress.

[K2-K6]

Didn't put enough detail in last post but the question about how big a skim has an impact on cam chain tensioner also as the total shift is doubled (2 runs of chain, up and down) so the tensioner will reach max tolerance earlier in the chain wear cycle.

Be interesting to see how you get on as I've never worked on one with a skim to my knowledge.