Topic: 74 CB750 - Unseized! Am I missing a part?

[Wingnut74]

So I finally got my pistons unseized and the head off, but am stumped on whether I am missing a part. I am thinking not but wanted to check. When I removed the head I did not see any rubber cylinder head stud packing studs mentioned in Hondaman's book, and from one of the CB750 parts sites (part desc pasted below). My VIN tag says it was made 11/73, but the title, bill of sale, all say it is a 74 K4.  I notice that the parts website excludes 73 from the bikes that it fits. Soo it must be that really some bikes made in late 73 that were K4s really dont need these? Os that correct? thanks

ps, I have to bore our my cylinders to +1mm, and scrounged up 4 Honda OEM pistons/rings (not cheap!). Does anyone know if the head gaskets that come with the gasket overhaul kits from the two big cb750 parts sites have big enough holes for +1mm? thanks!!



Cylinder Stud Packing Set (8pc)
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Cylinder Stud Packing Set (8pc)
8pc. set of rubber cylinder head bolt (stud) packings (short tubular rubber seals that fit over dowels) - OEM Ref. #12115-300-000.

Fits: Honda CB750K (1972, 1974-1978),
CB750F (1975-78) Supersport,
CB750A (1976-78) Hondamatic
Part #: 18-7018

[oldhatt45]

Wingnut74

Don't know for sure, but my K6 was manufactured in 8/75 and it is definitely a K6.
So, your K4 being manufactured in 11/73 is probably right that it is in fact a K4.

Hopefully Hondaman will see this post and be able to give you a more definitive answer or possibly one of the other experts will chime in with more info.

Charlie

[Davez134]

If your engine came with them from the factory, it should be obvious where they should be. If the only recessed area is around the oil passages for o-rings and rest of surface is flat then you don't need them.

[Jerry Rxman Griffin aka MuthaF'er]

Your engine serial number should help with the search in a parts manual or online.

[Davez134]

If your jugs look like this between 1/2 and 3/4 then it was designed to have those stud packing parts you are talking about.

If they look like this then it wasn't.



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[Dream750]

Just to add ... refer to this service bulletin to further clarify the two sealing styles based on engine numbers:

[Wingnut74]

Thanks for posting that service bulletin. I am stumped though because I have the o-ring style, but I am nearly certain that there were only 2- orings around those middle oil supply holes and definately not 8 o-rings as shown in the bullletin, and was untouched from factory - VIN 11/73 and sold as K4.  I think the two is correct thogh. Does this sound right that some bikes only used 2 b/n cylinders and head? thanks

[Don R]

If it worked before, it will work again. In theory.

[Dream750]

Thanks for posting that service bulletin. I am stumped though because I have the o-ring style, but I am nearly certain that there were only 2- orings around those middle oil supply holes and definately not 8 o-rings as shown in the bullletin, and was untouched from factory - VIN 11/73 and sold as K4.  I think the two is correct thogh. Does this sound right that some bikes only used 2 b/n cylinders and head? thanks

My 11/73 dated K4 originally had just the two cylinder head o-rings and still does after performing a stock top end refresh. The service bulletin is saying that a modified cylinder head gasket kit (12020-300-305) which includes the additional eight o-rings is available for an (optional) upgrade. At least, that's the way I read it.   

[PeWe]

About the o-rings. Same head gasket with 8 wider holes.
lThe later cylinder blocks and heads use the special pin with higher o-rings, the earlier without counterbored holes have o-rings only.

Very good information since I have the later (K6). Now with Cometic fiber gasket that have very small holes, no o-rings and  special pins. Oil leak yes. Maybe due to no o-rings around the 8-holes.
I'll use the thick o-rings + pins this winter job since I have a gasket made for it.

I have however, an additional old set, cylinder and head with no counterbored holes. This set will get the newer type of gasket and the o-rings like 12115-300-305.

O-rings
Size? Diameter as the high o-rings must be OK.  Thickness: is 1.5mm enough?
Quality? Viton, nitrile?

[IceFishingguy]

First off - this article and picture of heads cleared things up for me! Thanks! I have a CB750K3 that I am rebuilding and I have gotten to this same point as PeWe. Upon assembly Head gasket, I noticed in my parts bin from disassembly that i have 8 flat gaskets about the same size as the oil passage o-rings but these are flat. As a newbie and idiot, i don't have a picture of where these came from? Any help? I can snap a picture when I get home.

Thanks! Pat from MN