[Supras] cam journal caps

[Jeff Mohler]

Yes Bernie.


ALL of us have whacked totally out of spec cam bearing journals.


In fact, there are many millions of Yamaha heads out there, worldwide,
on countless Toyotas (and others) with the same condition as the
average SOGI member, including as poorly as yours were.

I believe it's anticipated into the design as the design is not
servicable, and is....acceptable.


I think Ive read of 1, or two...complete failures of the top-end of a
Supra head since 1996 on this list.  And at least one of those was due
to contamination from a dying crank bearing.

On Tue, May 6, 2008 at 10:07 AM,  <berniek at technicaldevelop.com> wrote:
> Dear Jeff:
>
> You are a well respected member of the list with quite of bit of racing
> experience and resourcefulness about brake parts, and probably much else.
> You have apparently learned by doing, a perfectly viable way of gaining
> education and knowledge.
>
>
> That does not work to the exclusion of viability of findings of others.
> For years I had a startup knock in my essentially stock '90 which I knew
> came from the top end, yet it made no sense.
>
> Only when I tore down the head from the relatively low mileage JDM did I
> find that the last exhaust cam journal had a diametral clearance of .006",
> rather large for a journal that is only about 1.3" in diameter and turns
> relatively slowly.  Others were worn as well.  Would you leave it in that
> condition?
>
> I would not.  The reason for cam journal scoring emanates from lack of
> boundary lubrication, let alone hydodynamic wedge formation.  I mentioned
> that very subject as covered in Shigley, a noted mechanical engineering
> text, and will not go into the details again here.  If you reduce the
> clearance (fortunately easy to do), the problem disappears.  Once the wear
> starts again, it should be attended to because the wear is regenerative,
> expecially for the last exhaust journals.  That is one reason the MKIV
> feeds the center journal, not the front journal with oil supply.  All of
> the journals are cross drilled, so little oil gets to the last one in the
> III once the cap wear progresses.
>
> In the final analysis, we need to maintain mutual respect.  You were
> correct about some of my street shenanigans and I accepted your words in
> that regard.  By the same token, accept what others are correct about as
> well.  Others have followed the procedure I layed out and have been
> satisfied with the results.
>
> Bernie
>
>