[Supras] cam journal caps

[Jeff Mohler]

PS:

I reserves Bernies right..to be right.

Just I wouldnt spend the $ unless it was just clear out bad..eating
itself alive, and then Id probly consider getting another head.

On Tue, May 6, 2008 at 11:19 AM, Jim Jobe <jjobe2 at supratech.org> wrote:
>
> Actually we've found quite a few that were worn out of spec often with
> the accompanying top end knocking after a BHG repair.  The cause is
> either oil starvation (cam journals are the last to receive oil) or
> milling a warped head flat vs straightening it.  Some of the
> aftermarket cams have come in with undersized journals too.
>
> While it is true that most of the force will be towards the cap,
> ideally you need the appropriate clearance around the entire
> circumference for proper oiling and support of the cams as
> Bernie stated.
>
> On Tue, May 06, 2008 at 10:51:31AM -0700, Jeff Mohler wrote:
>> 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
>> >
>> >
>>
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>