K0-K3 series 750's head gaskets v K4 onward - and head gasket leaks

[the-chauffeur]

I'm mid way through my third top end teardown because I've found another leak up there.  Turns out that it's partly due to one of the front studs that holds the cam towers in place not having a sealed end.  Threadlock will cure that, but while I was there, I had a look at the head gasket because I thought it had been seeping - not a huge amount, but enough to put a few unwanted stains on the fins around the head gasket.  I wasn't wrong . . .

 . . . until recently, I'd never really thought much about the differences between Honda's own head gaskets, head gaskets that come in kits and/or the changes between K0-K3 models and the K4's onwards.  I rather niaively assumed all head gaskets were created equal and that when kits proclaimed their gaskets were oil resistant, that meant they wouldn't leak.  Well, that's something I was wrong about - the same misplaced belief that what are advertised as complete gasket kits  come with all the gaskets and o-rings needed for engine rebuilds (yup, wrong again).

Here's where the gaskets differ. This is a gasket for the earlier year bikes - and this is one for 1975-ish bikes onwards.  Ignoring the centre channel section shape, check out the stud hole sizes.  On the earlier one, the holes around most of the studs are pretty small, and the gasket is made from a metallic coated material.  The later/aftermarket gaskets have stud holes that are wide enough to accommodate rubber joints; the material (certainly in aftermarkets) is largely fibre.  The inner stud holes are important because they act as oil returns (front and back), so it's important they seal properly to stop leaks.  The gasket I've just removed has the bigger holes, and because mine's a K2, there's no room for the rubber seals that fit later bikes.  Consequently, mine was covered in oil that's leaked out and around the mating faces.

So this time, I've managed to get hold of a Honda gasket that sits somewhere between the two above - it's the same material as the earlier one, the stud holes are wider but the kit also contains very thin square section rubber o-rings for 8 of those holes.  It was made for the US market in the '70's in an attempt to cure the head gasket seep problems.  Whether it'll work or not, only time will tell, but it's a hell of a lot more substantial than all of the aftermarket kit versions. 

If you have an older K series, I'd strongly recommend you try to get hold of one Honda's earlier gaskets with the smaller diameter stud holes (the image above lists the part number, so you should be able to order it from Silvers, CMSNL or Honda direct).  Although they're more expensive, they're significantly better quality than any others I've seen.  And don't do what I did and use one of the later aftermarket gaskets with the bigger holes on its own because they simply don't seal the oil return ways properly.  Once they start to seep, no amount of torquing down will cure the problem.  Having said that, the thin o-rings may be available from Western Hills Honda in the 'states, but they seem to be about the only place that have any stock left (part no 12115-300-305).

[Bryanj]

How about measuring the ID and thickness of the "O" ring you have, i know its square section but a normal round one might be better than nothing mate and the sizes would help others

[the-chauffeur]

The measurements (approximate) I got were:

Outer diameter - 17mm
Inner diameter  - 14mm
Thickness         - 1.5mm (make sure they're the same thickness as the gasket)

The square section o-rings fit snugly inside the oil returnway holes in the gasket.  Bear in mind that if you use round section o-rings, they're unlikely to fit quite so closely and will squish differently to square section types.  Also, there's some talk on the US forum about the material these rings should be made of - the general consensus seems to be viton because it stays elastic under heat and pressure.

One other thing to note with earlier bikes - the 2 o-rings that sit around the oil feed holes between the cylinders and cylinder head are also square sectioned.  Unlike the others, these larger rings are still available from Honda - part number for those is 91303-035-000.  These are thicker than the other 8 (maybe 2mm instead of 1.5mm) because they have recesses to sit in.

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I had the foresight to install one of the Gordon frame kits (see the US site for details) and so I can strip the top end in the frame.  Doing the strip and rebuild properly (from complete bike down and back up again) took me around 6 hours.  If you've got to take the engine out, figure another hour or two into the equation.

[the-chauffeur]

Much like the 750 gaskets, there appears to be a significant difference in the quality of the gaskets available (or that have been) for the 350 K series bikes.

As with the 750, I've been having oiling troubles with my two SL350's.  The problem emanates from the rear outer cylinder stud joint between the head and the cylinders.  The rear outer stud holes double as oil feed lines to the top end, so if the joint between the cylinders and the head isn't 100%, they seep oil.

Most of the Honda 350 gaskets you'll ever see are very like the OEM versions, but there seem to be subtle differences - and I think it's some of these subtleties that have caused the leaks I'm experiencing.  I've got 4 different head gaskets to compare; one from the sets David Silver sells (made in Thailand), one from an old Noboru top end kit (Japanese made) and an older Honda NOS.  All of the head gaskets are made of fibrous material with metal fire rings.

The Silver's kit gasket is the only one that doesn't seem to be coated with some sort of thin plastic finish.  It's 1.3mm thick at the edges, but the fire rings are around 1.5mm thick.  The Noboru gasket is around 1.35mm thick at the edges, with the fire rings being about 1.45mm thick.

I know the fire rings crush, but I can't help feeling that a thinner fire ring will give the rest of the gasket a better chance to flatten.  But I've also got another gasket I've been reliably informed is a used aftermarket which has two larger holes with inset rubber rings around the rear outer stud holes - not unlike the 750 gasket arrangement mentioned above.  Here's a picture:



Unfortunately, I don't know the manufacturer - which is a real shame 'cos I could do with a couple more of them.  I have a feeling the rubber rings would help enormously with the oil seeping.

[the-chauffeur]

Back to the 750 head gaskets . . .

 . . . taken from another post, here's a picture of 2 x 750 head gaskets:



The lower gasket is a generic (Cycle-X) 750 repro - it should be suitable for K4's onwards because it has the larger oil return holes designed to work with rubber seals around some of the cylinder studs.

The upper gasket seems to be from an F2 - 4 of the large diameter holes for the rubber collars are not there - they are smaller diameter.  It's genuine Honda as it has the "varnish " around the chain hole and 2 of the stud holes.  Another difference between genuine and aftermarket is that the top (Honda) gasket has the oval fire rings which are as per Honda, the pattern one has circular fire rings.

[the-chauffeur]

To add another facet to this thread . . .

 . . . I may have mentioned I've long suffered with a seep/leak from around the front left outer joint between the cylinders and the head on my K2.  Having tried a number of gasket combinations, together with heavy duty studs and various types of sealant, I still seemed to be getting nowhere.  And then I found a post on the US forum buried deep within a much longer conversation - clearly I wasn't alone in having suffered this problem after rebuilds . . .

"Early 750 head leaks from around the front edges?  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 the same parts as 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."

I've followed the microfiche fairly closely every time I've reassembled, and it turns out that was kinda my big mistake.  At some point in the past I replaced my locators with the 'correct' ones for a K2 - the 22mm ones - and they're too tall. When sat in the top hole, there was 11mm showing and when in the bottom, there was pretty much the same, which suggests that best case scenario, they'd allow for a 1mm thick gasket to crush flat.  But then I put the cylinder head on top of the barrels with the dowels in place and there was a gap that was more than 1mm all the way along the front.  Given that the head gasket averages 1.30mm thick, that's not a lot of crush. 

The most recent disassembly has left me with a 'used' head gasket that looked pretty much the same as it did before it went in - there was very little noticeable pressure on the fire rings and it was still the same sort of thickness around the front holes when measured against a new one (whereas the rear edge seems to have compressed by about 0.5mm).  As always, there was a thin layer of oil on the head gasket and yes, it was the cause of the leak at the outer left (it was only a matter of time before the outer right went the same way).  There was also a carbon lip on the cylinders from where I assume air/oil has gotten in via the wrong entry points - that came off with wire wool.

This time, I'm using the later dowels, which will allow me to get a proper crush on the gasket.