Topic: Head Gasket Leak

[The Lone Builder]

Hi All,

After several life events, and a foray into a British bike - frustrating or what! - I'm back to my Baby.

This particular baby is the CB750 K2 (with a K4 engine - 2310***) that I rode from Belfast to Belfast (25,000 kms) a couple of years ago. It leaked a little from the cylinder head en route, but nothing that I worried about. To cut a long story short after I rode it from Ireland down to south of France last year, the trickle became a stream, so I had no option but to park it up.

Now I'm trying to sort it out, and I have some questions I'd be grateful for comments on.

Everything came apart easily, which was not too surprising as the engine was rebuilt completely prior to my trip. Rocker cover gasket was in one piece and there was plenty of oil about, which I reckoned was a good thing. Some oil had pooled in the recesses between the valves as shown in the first pic.

The oil itself was clean enough, but there was a deposit of thicker, blacker oil sticking to the head; is this normal or does it indicate something amiss?

Taking off the head, I felt that the 4 lower 6mm head seal bolts were not as tight as I would have expected - maybe I omitted to tighten them down last time. Some of the head stud nuts also felt a little loose. I thought I had found a possible reason for my leak, or a least a major contributing factor, but then on taking the head off I discovered what is shown in the second pic.

The gasket was quite brittle, especially around where all the breaks are. I also noticed the large holes in the gasket around the studs and some, rather confusing, research, including SB 42 revealed that with this particular gasket probably needs some o-rings in those large holes. Any thoughts on this and the state of the head and pistons in general are welcome

Now I have a Versah gasket set which contains a head gasket - P/N: 12251-300-070 - and 8 things, called "cylinder stud gaskets" - P/N: 12115-300-000. The latter is not the P/N given in SB 42 - it ends in 305. have I now got the right combination?

Finally any thoughts on the state of the valves as per pic 3 would be welcome. I don't really want to do any major work here; I need the bike back on the road in a week or so.

Thanks for your thoughts.

[PeWe]

Did you tighten the head again after 24 hours or so last time?
All 16 nuts + the 4 center M6 + front and rear M6 after the nuts.

Some fiber gaskets compress rather much. Base gasket compress too + the o-rings.

[MauiK3]

Be sure to change the rubber pucks
Yes, retighten after at least 24 hours.
The fasteners won't feel that tight, especially the small ones.
Put plugs in and leak check the valves just for peace of mind.

[The Lone Builder]

I can't honestly remember. This was my first rebuild since 197??? and my first major rebuild ever! I was pretty meticulous - thanks to my position on the autism spectrum. You live and learn; I'm just happy it survived this long.

I will make sure everything is done "by the book" this time.

Another question occurred to me - the base gasket. I seem to remember reading that when removing the head the base gasket gets disturbed also and thus should be replaced. Does anyone else subscribe to this notion?

[The Lone Builder]

Be sure to change the rubber pucks
Yes, retighten after at least 24 hours.
The fasteners won't feel that tight, especially the small ones.
Put plugs in and leak check the valves just for peace of mind.

Rubber pucks on the way as they are not included in the Versah gasket set. The ones I removed were still sealing well, but there were all a bit stiff.

[david 750f]

I can't honestly remember. This was my first rebuild since 197??? and my first major rebuild ever! I was pretty meticulous - thanks to my position on the autism spectrum. You live and learn; I'm just happy it survived this long.

I will make sure everything is done "by the book" this time.

Another question occurred to me - the base gasket. I seem to remember reading that when removing the head the base gasket gets disturbed also and thus should be replaced. Does anyone else subscribe to this notion?

I have a similar question. Should the base gasket be changed if only the head is removed?

[PeWe]

I have removed/swapped heads without replacing base gasket that was rather fresh. I did not lift the cylinder block.

Might be different if it has been sitting there for many years and much use.

[The Lone Builder]

I'm struggling a bit here!

I have a Vesrah gasket set - 157/557 - which is for K4/K5 engines according to the label.

The head gasket has the bigger holes, which, from what I have read, requires o-rings to ensure a leak free seal. These o-rings are not included.

I saw this post http://forums.sohc4.net/index.php/topic,185208.msg2144565.html#msg2144565 in which Trigger showed the hole size to be 17,86. He didn't unfortunately show the o-ring size. The holes in the gasket I have are 17,7 - allow for analogue caliper and my eyesight.

Am I correct in assuming then that the o-rings should be 12,5x2,6? This would make them a little thicker than the 11x2,5 in the centre.

Omission of these on my rebuild probably was the main cause of my oil leak, so I want to do all I can to get it right this time.

Thanks.

[PeWe]

There is a Honda Service bulletine about later head gasket for earlier cylinders.

Which o-ring size to use in all 8 oil return holes.

Found my own comment about late gasket with 8 wider oil return holes in gasket.
New gasket is probably 1mm thick so thicker oil feed gasket can be needed too if cylinder is not milled.

O-rings ordered from Bearing shop UK
Oil return holes 8x
ID 12.5mm, thickness 2.0 mm

Oil feed for the thicker head gaskets.
ID 10.78mm, thickness 2.62mm
(Honda stock 11x 2.5)

[The Lone Builder]

Thanks PeWe,

I saw that post.

The gasket is 1,6mm thick so does that require an even thicker o-ring? 3,1???

[PeWe]

Really 1.6mm thick head gasket?
The outer ends, not the metal rings around the bores.
I commented o-ring thicknesses vs gaskets in another thread.

I searched for o-rings info and found the recommended o-ring compression to be 20-30%.

This showed that Hondaman is correct about stock o-rings are for stock head gaskets of 0.8mm thickness. Oil feed o-rings.

Thicker gaskets around 1mm needs the thicker o-ring to keep the compression ratio.

The oil feed o-rings have a lower area in the cylinder block. I measured several I have, around 0.95-1.0mm deep.
So a 1.0 mm thick gasket + this will make 2mm.
The o-ring must be thicker than this so it will be compressed 20-30%.

When calculating I found that there is a limitation of o-ring thickness when the outer dim is restricted by 16 mm. That is the machined area in cylinder block.
If milled, these "holes" must be milled too so same depth is maintained.
If not o-ring must be thinner if max compression is exceeded.

If going thicker the inner dia will be reduced and enter the hole where oil flows.

I have seen that too thick o-rings under cam towers are not good either. O-rings can crack. Happened to me, small viton fragments close to block the oil orifices.
That was viton o-ring that might be harder than nbr (nitrile).
I found stock o-rings I used. I have bought stock size nitrile after that. This for the later heads with smaller orifices.

The other thread with o-ring info with my calculation examples with link to the o-ring comoression info
http://forums.sohc4.net/index.php/topic,153549.msg2203559.html#msg2203559

I have earlier attached the head gasket Service bulletine.
This for the other 8 o-rings for oil returns when using later gasket for earlier cylinders.

[The Lone Builder]

I think I'm now totally confused - certainly unsure! Let's see if I have it right:

There are 2 o-rings here:
     The two at the back for the oil passages - P/N 91309-035-000 or 91310-300-000 - measuring 11x2,5 according to the parts fiche.
     The 8 "new" ones to fill up the bigger holes in the gasket. The part number for the set of 8 - P/N 12115-300-305 according to SB #42. No size is given, but according to another post I found – Reply No 6 – the size is

Did my best at measuring.

17.4mm outside diameter
1.35mm thickness
these are square section (not round)

David

Now my head gasket is 1,67mm thick – yes it is; the old one was 1,48! So that said I need to increase the thickness of the o-ring by 0,8mm, given that the original Honda gasket was 0,8mm?

This means I need 2 of 11x3,3 and 8 of 10,8X3,3 with a square section. Is that correct?

One last question, would x-rings work in place of the 8?

[bryanj]

That is the out of the packet size, what about compressed?

[The Lone Builder]

Well finally everything is buttoned up and the engine is running - no leaks so far.

I heard an unusual?? ticking at the exhaust and I thought there was some smoke coming out the back so I took this video.
https://youtu.be/gVhCCgHla2w

There is no smoke, but is that ticking normal?

[newday777]

Well finally everything is buttoned up and the engine is running - no leaks so far.

I heard an unusual?? ticking at the exhaust and I thought there was some smoke coming out the back so I took this video.
https://youtu.be/gVhCCgHla2w

There is no smoke, but is that ticking normal?

The carbs sound out of sync