Why it would be a really silly idea to use Yak Fat in an 8V (English)

OK, so I've got a warranty job coming in. A bloke with an 8V Griso with a pinhole pourosity in the head casting so it's a new head for him. This actually arrived within a fortnight, I was very impressed. while I had a new, clean, head in my grubby little mits I tought it would be a good time to have a squizz at it and find out what's what.

Now we know from the publicity blurb that the engine is Air/Oil cooled and has two oil pumps, one for lubrication and one for cooling. Certainly when the motor is running large amounts of oil gush out of the camboxes and rockers and flow down overe the head to cool it. Run an 8V with the rocker cover off and you'll be drenched in oil in seconds!

Having the head off the bike and clean though allows you to see some other much more important things.

As you can see there is a gallery drilled through the casing by the exhaust valves. There is another one that comes in from the port side of the valves as well.

As you can see there is a gallery drilled through the casing by the exhaust valves. There is another one that comes in from the port side of the valves as well.

Here you can see the two plugged drillings in the casting for the lower gallery in the bottom LH corner of the pic.

Here you can see the two plugged drillings in the casting for the lower gallery in the bottom LH corner of the pic.

You can see here the boss the gallery is drilled through in the casting.

You can see here the boss the gallery is drilled through in the casting.

Looking at the underside, (Combustion side.) of the head you can see on the exhaust side, (Bottom of the picture.) the two drillings, presumably one a delivery and one a return, in the head. They are the two smaller holes adjacent to the stud holes. The longer slots are part of the oil return for the lubricant that gets flung around and pumped out of the cam boxes/rocker gear above the head.

Looking at the underside, (Combustion side.) of the head you can see on the exhaust side, (Bottom of the picture.) the two drillings, presumably one a delivery and one a return, in the head. They are the two smaller holes adjacent to the stud holes. The longer slots are part of the oil return for the lubricant that gets flung around and pumped out of the cam boxes/rocker gear above the head.

Another pic looking at the exhaust port showing the second drilling for the cooling oil feed.

Another pic looking at the exhaust port showing the second drilling for the cooling oil feed.

Looking at the valley side of the head you can see the lubrication oil feed. That is the hole at the front of the head in line with the exhaust stud. This feeds oil into the upper forward stud hole.

Looking at the valley side of the head you can see the lubrication oil feed. That is the hole at the front of the head in line with the exhaust stud. This feeds oil into the upper forward stud hole.

Bottom right in this pic, where it is fed up the stud and into the cam bearings in the cam box.

Bottom right in this pic, where it is fed up the stud and into the cam bearings in the cam box.

As you can see from the pic that stud is doweled to the barrel to ensure that there is room for the oil to flow up to the bearings through the stud hole when the stud is centralised by the cam box casting.

As you can see from the pic that stud is doweled to the barrel to ensure that there is room for the oil to flow up to the bearings through the stud hole when the stud is centralised by the cam box casting.

The important thing here though is that the exhaust valve seats and the head immediately surrounding them is being cooled by a high volume of oil that will be being pumped through the galleries surrounding the seats. The temperatures in tat area are going to be the highest in the motor and using an inferior oil is going to lead to all sorts of horrible varnishes and deposits building up in those, quite small, galleries. when the factory recommends a very expensive, high quality full ester synthetic for the 8V it's not so much for the tappets, (Although protection there is a bonus.) it is because of the oil's ability not to degrade at much higher temperatures than a 'Free Grandfather Clock with Every 4 Quarts' mineral oil from Wallyworld.

Ignore the spec at your peril, and yes, any half way decent mechanic will be able to see the deposits and know what they are if the galleries do clog up and you drop the head off a valve. (BFG. This is particularly relevant to you as the 'tard who has been working on your bike has been swearing by Yak Fat. I'd drop it out yesterday, if not sooner, and put something better in!!!!) If I got one in with that sort of problem I'd be sending an oil sample off for analysis and if it wasn't the 'Good Oil'? Bye-Bye warranty!

Pete