[Supras] cam journal caps

[berniek at technicaldevelop.com]

I fully agree that in an ideal world, we would have round cam journal 
bearing surfaces, with perhaps a bit of recess around the parting line 
(evident in bearing inserts to promote hydrodynamic wedge formation).  
However, close examination of a worn cap will indicate that about 3/4 of 
it has been polished away over the miles since the valve springs push 
the cam up into the caps.  That wear is round as can be determined from 
a surface plate setup with thin shim stock, a cam, cap, and dial 
indicator on a stand to show slop off the vertical axis.  I really 
believe we should be glad that for once some aspect of repair is 
relatively easy in our beloved beasts.  The "half bearing" concept is 
not new.  Propeller shafts in large ships are supported on the bottom 
only, often in hardwood (lignum vitae) scraped to fit, fed with oil, 
while the top of the shaft is covered with canvas. 


Since the head half of the journal does not wear, refinishing the caps 
by parting surface material removal does indeed bring the journal 
support back to being nearly round. 


What makes me scratch my copious bald spot is that synthetic oil (at 
least Mobil 1) does not appear to make a difference, at least in the 
long run.  I dunno, maybe dino oil would have made it wear faster.  My 
former '90 developed the top end startup knock at about 120K, and 
ultimately blew the HG at 156K.  Then came the '91 from Houston, and the 
build up.  But the cam journal clearance will be watched. 


    Jeff:  What ever happened to the oil sale?


    Bernie

'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 Jobe 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.
>   
(balance snipped)