Topic: 750 K2 Head Gasket Leak

[mal]

My 750 motor has the engine number CB750E-2350XXX. This was one of the ones before Honda introduced the "sealed cylinder stud knock pins" (as according to Honda Service Bulletin # 42) to reduce head gasket leaks.

12 months ago I rebuilt my engine, and it was put back together  using a Cycle Exchange gasket kit. The engine was completed and filled to the top with oil to ensure there was no corrosion on the internals. It has not been run since the rebuild.

The problem is that there is an oil leak (minor but still there) along the front of the head gasket.

I know its highly unlikely, but is there any chance this leak will disappear once the motor has been run / bedded in?? I know I'm clutching at straws, but I really dont want to tear it down again unless I really have to. I was wondering if the gasket material that surrounds the "cylinder ring" might bed in and seal the leak?

Now before you chime in with the obvious about correct head bolt tensions, and using new O rings, yes Im sure the tensions were correct, and I used all the bits supplied in the gasket set (apart from the 8 rubber knock pin seals that were not relevant to my engine).

The only thing I am kicking myself about now is not having installed heavy duty cylinder stud bolts when I had the chance. I figure the problem would probably not have occured if I had used them.

Any suggestions greatfully accepted (keep it clean!!! )

Mal

[andy750]

When you say filled to the top with oil how much oil did you actually put in there? Usually you would add a quart to the head (through one of the tappet covers) with the bike on its sidestand to get some oil to the head. Its hard to see why it would leak now if its not been run as there is no oil pressure. Suggests the gasket is not sealing. Maybe someone will chime in and tell you it needs to run in order for the gasket to be seal....i dont know that for sure but would seem unlikely.

Also where is it leaking from exactly?

cheers
Andy

[mal]

Andy

It was literally filled to the top as I knew it was going to have to sit for a year before I could get back to it.

The leak is along the front of the gasket just under the exhaust outlets, but its hard to tell exactly where as the gasket looks wet along most of the front of it.

Mal

[markb]

I have a theory about this.  When I've torn my engines apart the knock pins from the cylinder to the head were really hard to remove and I had to destroy them to get them out.  Not sure what size they were (I think they were 20mm) but if you check the parts book the pin is called out as 22mm long but is no longer available (I think it was a mistake).  Later models show a 18mm long.  When I checked the depth of the hole in the cylinder and the hole in the head and the thickness of the head gasket it totaled 20mm.  In otherwords if you have 20mm pins, you bottom out in the bottom of the holes without adequately compressing the gasket.  Therefore the pins are jammed in making them hard to remove.  I believe that's where the later models used the 18mm pins.  I've used 18mm pins on my rebuilds and so far no head gasket leaks.  I did add the APE studs and the Bulletin #42 o-rings so maybe that did it but I see no down side to using the 18mm pins.  Like I said, just my theory.  Its just strange that most of these engines leak and even stranger that yours leaked without any pressure.  I going to tear down another motor soon and I will measure the pins.

Update: Got the motor apart and the pins were 18mm long.  Both motors had been apart so really its hard to say what was originally installed but I will stay with the 18mm pins.

[mal]

Thanks Mark

I'm assuming you are referring to the dowel pin length here, as I am reasonably sure there is no knock pins in my motor (I'm dredging up the grey matter here, the memory aint what it was )

You are right about how difficult the dowel pins were to remove, and maybe that is the cause. Trouble is, the answer entails lifting the head - DAMN!!!

If no other suggestions come to light, that will be the only option.

Mal

[Toxic]

Sorry to say, It's not going to seal itself.  But HD studs would not have prevented it either.

Torque is torque wether it is applied to a HD stud or a regular stud.  so at least you shouldn't have that regret.

[MCRider]

Sorry to say, It's not going to seal itself.  But HD studs would not have prevented it either.

Torque is torque wether it is applied to a HD stud or a regular stud.  so at least you shouldn't have that regret.

But the HD studs accept about 10ftlbs more torque than the standard studs, which would turn to taffy at those torques.

IMO, and experience, assuming no problem with the overly long knock pins, the HD studs will definitly prevent head gasket leaks.

As to the knock pins, if there are truly none, you are asking for problems unknown. They are there to assure alignment and are in every motor, between the case and cylinder, and cyl and head.

[markb]

I might be wrong but I don't think mal is saying he doesn't have any pins.  He just calls them dowel pins instead of knock pins?

[MCRider]

I might be wrong but I don't think mal is saying he doesn't have any pins.  He just calls them dowel pins instead of knock pins?

Could be. terms are always a problem. But that needs to be clarified. Without them the case/cyl/head alignment could be off.

[HondaMan]

Like MCrider said, there were (or should have been) knock pins in the cylinders. The "change" was to longer dowels and long, recessed rubber dowel seals to go with them.

Is the engine still full of oil, now? If so, it could have seeped up past the piston rings, into the bores, at which point it will leak past the metal compression ring which hasn't been pressure-sealed yet, and dissolved away the wax-like sealant on the surface of the head gasket between the bores and the front. I have seen this happen even on used engines, when they had been sitting for years on their faces, after sitting for years in the bike, during which the oil drained from the tank into the cases, after which the engine had been removed...

But, that means lifting the head...

I have noticed some gasket kits recently that literally take Honda's spec of the O-rings to heart: it points out how today's parts builders miss Honda's point of their O-ring numbers. For example, most of the O-rings in the oil system are 15x2.4mm in Honda's spec: this means the MINIMUM sealing thickness of the O-ring's girth is 2.4mm, below which it will not seal correctly. That O-ring was normally a 15x2.5mm from the Honda part bag...Athena's kit comes to mind, as this was the last one where I found this to be true: they used 30.8x3mm O-rings for the tappet covers (should be 31x3.2 or 3.5) and 15x2.4mm for the oil pump (which had to be hard to get!), which are visibly thinner than the ones that came out of the engine. I'm afraid there may be a lot of this going around... 

The Vesrah kits seem to be OK. I haven't seen a Cycle-X kit.

[markb]

+1 on the Vesrah kit.  Everything seemed to match and so far no leaks.

[mal]

Honda Man

It sounds like you have hit the nail on the head, as I poured oil in the cylinders as well, as a preservative, as I knew the motor was going to sit for quite some time without running. Damn, I had no idea that the oil would deteriorate the gasket if it seeped past the metal O rings in the gasket.

Oh well, you live and learn. Order another gasket......

Thanks for the help guys!

Mal