305 Head Gasket-What Caused This?

[jensen]

That's why I going to try, in the mean time go on with what you're planning to do
see it as a long term to be tested solution, not as the solution until pr oven otherwise

It's more that if this works, it's cheap and Honda

I'll go on with looking for a decent solution, also other diameter O-rings.

[davomoto]

That would be my concern as well. I've never had a problem with the stockers, so....... why fool with something that works?

davomoto

[jensen]

the standard O-rings Honda uses in there own gasket set has p/n 91303 259 000, and are measuring 11.0 * 2.0. These will work in most cases (like mine) in combination with the Honda gasket 12251 275 000 (for a 305). If you use another brand head gasket, there's the risk that o-rings and head gasket don't work together.

I understand that you did use ordinary o-rings, but for every application you need to verify the specifications, an O-ring isn't just an O-ring. There are a lot of materials where o-rings are made off like NBR, Eco, butyl, HNBR, EPDM, silicon, fluor-silicone, Viton®, Aflas™ en Kalrez® (including a few brand names). So it's possible that you used the right size, but that the material wasn't up to it's environment (heat, pressure) or the rings weren't resistant to the oil you use (chemical resistance).

For me it's not clear why the standard solution doesn't work in some cases, I only have experience with my own engines. But from people with a lot of experience like Loud Mouse, it's clear that the Honda solution isn't working always, and he found a solution to it (using bigger O-rings). Also Dave found another solution, (using Honda bond) in combination with the standard o-ring size.

In the S65 (and in many other 50, 65, 70 and 90 cc applications) Honda is using another technique.
They use flat rings (soft), but these rings can't compress out of the head gasket because in the cylinder there is a little chamber to fit in. The diameter of the rings can't get any bigger because this is prevented from a mechanical way.

I don't think that the S65 rings will work better than the standard O-rings Honda uses, but I will find out, but I don't ask you to try it.

Like I said there are many sorts of O-rings, I will call my O-ring specialist Monday (ERIKS) to see what they advise. I have very good experience with them (work) but also with almost all oil seals for the 250 / 305 engines I rebuild (except for the crank seals, these are to specific).

Jensen

[jensen]

yesterday I looked through a few NOS head gasket sets from Honda and other makes (all for CB72)

I have at least 3 different Honda head gasket sets and within two sets 268-020 and 268-030 I discovered a few different items, also the O-rings. in the 020 set these rings are black, and in the 030 set these are green.

just letting you know that also Honda had more then one type O-ring, but not a change in part number of this O-ring and that's making me curious. The green material seems more elastic to me than the black ones.

I always used the green ones, and no problems (without using bond) ever.

(if anyone wants I can post pictures of the different O-rings)

jensen

[jensen]

I measured both O-rings, the black ones (0611-268-020) measure 14.8 mm O.D, and diameter 1.95 mm. The green ones (from 0611-268-030) measure 15.0 and have a diameter 2.1 mm. The green one is softer than the black one.

My part manual says p/n 91303-259-000, size 11 mm I.D. with diameter 2.0 mm.
as far as I know only one part number exists, and that's this one, but like I stated in my previous mail, it are two different parts (color and size)

It may seem that the difference is to small to mention, and maybe that is the case.

[teazer]

I think that you correctly diagnosed the problem. use stock small cross section O rings. Anything much larger has no where to squeeze to.

There's only a few psi of oil pressure and then there's the dowels, so leaking should not be an issue.

Assemble dry - no gasket goo and it should last forever (more or less)