Head / barrel oilway o-ring

[Geordie]

175 Mototrans ?

..

125, 175, 200 Mototrans are listed the same dimensions. Bought 2 Viton today at the local hydraulic shop.

[Geordie]

The head has been off twice more and two o rings used. The result has been the same - oil at the head/barrel joint. The o ring was found pinched around its periphery at each removal.

However, as I found the head bolts to be oily as well as the machined alloy surface surrounding the liner, I'm pretty sure the o ring is a red herring.

IMG_barrel_1.jpg

There is a gap between that surface and the corresponding one on the head of 0.3mm. As the bolt hole overlaps that, oil escapes at all four driven by crankcase pressure. Oil passes up the root of the crankcase and bolt threads.

IMG_20220307_080357_842_1_1.jpg

Or am I barking up the wrong tree?

[Jordan]

I'm not sure, but if oil does make its way up the head bolt threads, some non-setting sealant could stop that.
Are you using Viton or silicone O-rings?

[ducwiz]

An O-ring is designed to be deformed in situ not more than 20% of his thickness, otherwise it will be damaged, and a leakage is the consequence. So, take some piec of modelling clay, wrap it around the dowel, put the head back in position and tighten it slightly. After removing, you can measure the remaining thickness of the cla, which indicates the width of the gap in the O-ring area. If it is less than 1.4mm, one or both frontal faces (head and cylinder) have to be machined down in order to achieve the correct gap width.
And of course, use a Viton/FPM grade OR.

Hans

[Duccout]

Hi Geordie,

What a pain. I do not think that your diagnosis is correct; I have been running and working on Ducati Singles and Twins for almost fifty years and have never found one to leak at the head joint. The only way that I can see that oil is getting there on your bike is past the 'o' ring or coming out somewhere else around the head area and finding its way to the joint.

I am not saying that it is impossible for oil to work past the threads of the head bolts, but if it were the case, then it would surely be a common problem? I don't know what to suggest, apart from checking that the head is not being kept from seating properly by the oil way dowel in the top of the barrel, or to try what ducwiz and Jordan say. Have you tried seating the 'o'ring on a smear of silicone? It should not be necessary, but maybe worth a try. Keep us informed.


Colin

[Geordie]

An O-ring is designed to be deformed in situ not more than 20% of his thickness, otherwise it will be damaged, and a leakage is the consequence.

And of course, use a Viton/FPM grade OR.

Hans

The OR is1.66mm thick and Viton.
In the countersunk it is proud by 1.0mm.
When the head and barrel are together, the gap is 0.3mm.
The OR compression is 1.0 - 0.3 = 0.7mm
0.7 / 1.66 = 42% but does not allow for the deformation into the countersink. What is occurring is deformation into the 0.3mm gap.
The countersink and the OR are both correctly sized. A better solution would be a counterbore to a depth of 1.66 x 0.8 [ > 1.33mm]

[Geordie]

I am not saying that it is impossible for oil to work past the threads of the head bolts, but if it were the case, then it would surely be a common problem?


Colin[/quote]

My thought exactly but I have searched this site and it's not. Assembly first was dry, next was lubed and last was with a little Dirko high temp silicone.
The use by Ducati of a countersink instead of a counterbore was not their best decision but it appears to generally work. That leads me to my diagnosis of an alternative route.

IMG_orings.jpg

Countersink and o- rings.

[Duccout]

So your recess is countersunk instead of counterbored? I have not come across that, all the heads that I have seen have been counterbored, so maybe that is the problem? You think that when you tighten the head the 'o'ring is not being pushed into the countersink?


Colin

[Geordie]

You think that when you tighten the head the 'o'ring is not being pushed into the countersink?

Colin

It does go into the countersink but not all as some goes into the gap tween head and barrel. The consequences are shown in the photo.

Hard rubber, although deformable, is barely compressible.

It could be a machining error from new but countersunk seats are discussed on other threads of this site

[Duccout]

It does not look good. I would like to hear what Nigel has to say on this, you cannot be the first person to run into this problem.