[Supras] cam cap fitting

[berniek at technicaldevelop.com]

Jim:


    Yes, you got it right.  Actually, the bearing cap to journal 
clearance winds up rounder than might be thought. I checked that using 
thin plastic shim stock, a worn cap, and one journal on a cam.  Lateral 
lash was measured using a dial indicator and stand on a surface plate.  
Don't forget that the force vector which pushes up on the journal is not 
always opposite to that of the valve spring.  There is a continually 
moving offset because of the lobes "wiping" over the shims, so the wear 
force is spread out in the cap to some extent.


    Just use plastigage in the fore and aft direction of the journal to 
determine if you are beginning to taper the bearing surface.  If so, 
correct it with more force on the abrasive cloth on the side with the 
most clearance.  Try for clearances of .002".  Do trial fits frequently. 


    If you do this in the car, remove the two caps adjacent to the one 
you are working on and reinstall them with a few thicknesses of paper 
shims to push the adjacent journals (and thus the one you are working 
on) down into the head saddles so you get an accurate cap clearance 
reading.  When doing the rear caps, just shim down the next to last 
cap.  You don't need to remove the plastigage when checking at 
intervals.  It will stick to the cap, journal or both, and go right back 
into place when you put the cap on for the next trial fit.  Cleanliness 
is very important when checking fit.  Just tighten the paper shimmed 
caps enough to insure journal seating in the head, no more. 


    Good Luck.


    BernieK
91T 5 speed, refreshed JDM, '98 MkIV wheels, Lexus AFM, 550cc ND 
injectors, Walbro, modified stock adjustable FPR, 57 trim CT26 (no 
turbine clip, but wastegate passage bored out), deflashed valve bowls, 
'98 mounts (strong!), Treadstone IC, hard piping, Lipp, 3" Random Tech 
downpipe and single cat, 80 mm HKS LET-T16 exhaust, Dawes ball and 
spring boost controller (for now, set to 17 PSI), modified Raptor 
recirculated BOV, 2 mm Cometic HG, ARP head studs, helicoiled exhaust 
stud tappings, chrome moly lightweight flywheel, Exedy organic clutch 
(street use).



Jim Wooden wrote:
> Bernie:
>
> Sorry, I have been traveling and missed the beginning of this thread...
>
> as I recall your solution was to "sand down" the cam bearing caps 
> until a plasti-gage reading
> indicated the correct clearance this gives slightly "egg-shaped" 
> journals but a far better result
> than "doing nothing"!
>
> Did I get it right?
> I ask because I'm about to put a new timing belt on my MkIII supra and 
> it would be a perfect time... ;-)
>
> Thanks,
> Jim
>
> original owner 86.5 MkIII NA
>
>
>
>> 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
>
>
>
>