Head / barrel oilway o-ring

[Duccout]

I'm wondering if an 'o'ring with a smaller cross section may be better?

[Jordan]

I thought to modify by a counterbore (square section hole) rather than original countersunk (conical hole) - as done on the twins.
But I gave up on that idea when closer inspection showed that this is not possible due to lack of space and available O-ring sizes.
Maybe someone else can do it, but meantime I'm making sure to use soft, high-temperature capable O-rings - so far so good.
When I ordered heat resistant rings in the past, what was supplied looked just like the usual black ones, weren't any softer and didn't seal for long.

[ducwiz]

Imho Viton/FPM rings aren't basically softer compared to NBR versions. Both are characterized by their Shore hardness, which can be equal for both - or not.
The Viton rings I purchase her in D are easily identified by their color: brown like coffee with a few drops of milk. NBR types are always black.

Hans

[Jordan]

Thanks Hans, I thought all Viton and silicone O-rings are softer than NBR - not necessarily so.
I like to use a soft grade, because I think they will deform without breaking apart.

[LaceyDucati]

Hi,
Colin asked for my opinion.......All I can say is I've been around bevel singles and twins for 30 odd years and I've never really had any issues with head O rings leaking. Now I'm really going to upset things by stating that I've only ever used Nitrile O rings as original (at least for the singles). I have noted that the original chamfer for the O ring on the head does vary a fair bit, as does the clearance between the aluminium of the head and cylinder. The only issues I have had are as follows:

1) One leak from a short stroke race engine, however the top fin had been removed and there was reduced area for the O ring sit on and it therefore pushed out. Some remachining and a special dowel with a flange, sorted that.

2) repeated leaks from this area on some Desmo's and 450's, none of which were related to the O ring. The desmo's were due to spindle bungs going hard or being disturbed, then leaking down the stud. 450's were due to breeches of the spindle hole to the bolt holes leading to the same result. The 450 leaks were either through wear/damage or flaws in the casting. Spindle hole breeches can be observed by putting fluids like petrol or brake cleaner in the spindle holes and see if it appears in the bolt holes. If it does it will leak oil between the head and cylinder.

3) Damage to the chamfer surface by heavy corrosion or mechanical abuse.

4) Early Misano heads that had a steep smaller diameter chamfer, that the designer said would work "better". However that just introduced a leak that had (to my experience) not existed before. The O ring became over compressed, but that said some did not experience any issue, maybe they were using external feed. For the heads I supplied at that time, I promptly re machined the chamfer coping the original design and the issue promptly disappeared as fast as it arrived.

Looking at Georges O rings I would think they would have sealed with the amount of compression. I have seen O rings compressed beyond those and squidged out excessively between the faces and they have still been sealed prior to disassembly. I would be looking for possible flaws/cracks in the aluminium of the head or cylinder. I can't think why some have had issues, I don't think I've been doing anything special. I do always lubricate the O'ring with oil before installation, but I would assume everyone would.

One other question would be excess pressure (undesirable) from a high volume pump in conjunction with other changes to the oil system. I haven't had any issues with our high volume pumps, but if used with other mods, it could be an issue. This is unlikely, but just putting it out there. At this point I would also add I've never had a leak up the head bolts and I think that it highly unlikely. If you had that much crankcase pressure it would be coming out the breather and no doubt other places.

Maybe I've been luck......

Regards Nigel

[Duccout]

I must apologise for my senility here, my memory told me that the Singles had a counterbore at the oilway, but I don't think that is the case. Don't get old...... Still never had one leak, that IS true.



Colin

[Geordie]

Thank you all for the responses above.

The engine is entirely standard which rules out some of the potential sources.
Viton may be an improvement over nitrile which was originally used but both have been satisfactory for others. It clearly has been compressed in my case and could be expected to seal.
If it did seal then a route would be via the head bolts which would explain the oil on each of them and around the gap between head and barrel. Or....
The oil was from a joint higher than the head/barrel, ran down and appeared on the forward end of the top fin of the barrel. Whereas.....
If it didn't seal, that oil wicked around the gap and down the head bolt bores. .

I will clean up all joint faces and replace seals to them.

[LaceyDucati]

If the oil is in more than one bolt hole and more towards the front, I would consider oil in the cylinder leaking out, either via the head face (poor seal) or out the exhaust thread (poor seal) post cylinder or via the exhaust guide. Is the top of the piston oily and is there any signs of oil in the exhaust port? Ive seen a few leaks in these areas over the years.

Nigel

[Geordie]

Thanks Nigel, but no. Dry carbon deposits to head, piston and exhaust. The oil leak was as clean as the sump oil which has 500km from half a dozen runs. Rebored, and new valves + guides (yours). Compression is v good. No broken rings. The head is now back on and torqued. I'm out of banjo washers so popping out.

[ducwiz]

The O-ring in question is called GACO OR107 HR in the parts manaual. I recall that I found somewhere in an old catalog that "HR" means FPM or FKM or Viton, whatever you like
Btw, the small O-ring which resides in a groove inside the valve guide in the desmo heads has the same suffix "HR". It seems clear, why the use of Viton is mandatory here.

cheers Hans

EDIT: a comprehensive O-ring cross reference compilation is offered here: https://www.thisoldtractor.com/o-rings.html

EDIT2: https://gacoseals.co.uk/ lists the mat. grades (bottom of page/left) but unfortunately gives no "translation"