I have spent some time on this as well.
First, you are correct to worry about this. Of all the specs on the head, the cam bearing clearance is the most overlooked, and sadly, the one area where alot of wear occurs, especially on the rear journals far from the front oil feed.
The spec from Toyota is amazingly loose for a 1 inch bearing (.0051"). On my car I was actually exceeding this on the rear exhaust journals. Bear in mind that the valve lash is only nominally .008" on intake and 010" on exhaust, so the cam bearing can add 50% more slop to the system making it very noisy if they are loose.
You can do this with the valves in place, but you want to remove the buckets and shims. The cam will then lie in the head at specific angles with no spring pressure on it, which is essential for accurate results.
Here are some detailed instructions to do this from Bernie on the supras.com mailing list.
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1. I've worked out a probable method by which this can be fixed in place without much trouble. It is offered in good faith without guarantees of no camshaft damage, although with closed valve spring loadings of only 50 #. it should not be a problem regarding camshaft damage. Rest the cams in the head with the buckets and shims removed for lobe clearance. Remove cam bearing caps and insert small squares of soft brassshim stock, possible .020" thick, and tighten the caps just until the Journals bottom against the alumimum saddles in the head, no further or you will distort the caps. DO NOT ROTATE THE ENGINE WITH THE SHIMS IN PLACE. Between the top caps and the journals, being careful to not score the soft journal coating if you have it. Then remove one Journal at a time, and plastigage it. The other shimmed Jounals will hold the one under test to the saddle. Use red Plastigage and determine the clearance. It may surprise you.
2) Holding a single cut fine file in a vise, run the bearing cap over the file in the cutting direction about 20 times, reversing it once. Clean the file teeth and try a new Plastigage fit. If you have small surface plate and dial indicator you can use it to determine approximate amount of material removed.
Filing is not recommended for main and rod caps due to changes in insert crush, but should be OK here. Retry the Plastigage and repeat filing to obtain a maximum of .0028-.003 clearance. Do the same for each of the bearing caps, restoring the shim to each. When all done, take out all of the
shims. The first journal where the seals are) will generally be found to be OK. You will notice less top end noise and possible higher hot oil pressure at lower speeds. Apparent valve lash will also decrease.
Try to get a maximum of .003", minimum of .002" (less is ok if there is no head warp, covered below). The TSRM allows .004" but that is really too much and (I believe) leads to startup knock for a split second. If the Plastigage shows a taper, try to load the cap with pressure on one (long) side during the next file iteration. Be sure to clean the file teeth each time. Use a good file, like a Nicholson.
3. You can do the same thing with a surface plate and emery cloth, as suggested by some others on the list. One posting (I forget by whom)
suggested holding the emery paper to the surface plate with a film of WD-40, which sounds like it should work. In my case, I did the original material
removal with a Bridgeport, and used a file for touch up, but all of the material can be removed with the file or surface plate if done in small
increments and checked frequently with Plastigage.
You will probably find the rearmost exhaust journals to be the worst. The combination of "mystery coating" disappearance (especially on the
rearmost exhausts) and less than the best top end oiling design increases journal clearance. No matter, the caps can be made to fit correctly. You
may want to put all of the caps on when "almost there" to assure that the cam still turns freely (oil the journals for this). In some cases, it is
plausable that some head warp will limit the extent to which you can approach clearances less than .003". I just got my head back from the
machinist after he removed .012" to clean up the gasket surface. He stated that there was about .005" twist in the surface, so it may affect cam
journal loading. Under oiling conditions with the engine running, I would not worry about it, though.
Also, I would not worry about increased clearance at the cap parting line. Most of the cap wear (the other contributor to looseness, aside from
coating disappearance) will be nearly cylindrical due to the cam being pushed up into the caps from the valve spring forces. The top end oiling in
the 7M engine is not the best, IMHO, so there will be more wear as the miles pile up. One consideration emanates from the fact that journal oiling is
through the hollow cams, fed in at the front journals (which should be OK regarding clearance, incidently). Centrifugal force pushes the oil out of
each of the journal oil holes, and that force increases as the square of speed. I believe that is the reason why the rearmost journals are worn the
most, ie, they are the last to get oil. So getting the journal clearance down will help prolong journal bearing life.
In my case I brought the head to the machinist bare. Having a lathe here with a generous spindle bore to clear the valve head, I can use a tool
post grinder to take material off the valve stem tips to set the clearance. If you don't have a lathe, install the cams in the head after the valves and
seats are reworked with the caps on, and stuff some stiff foam under the cams where you can to lift it into the caps. Install the buckets and shims
first. Put one valve at a time into the head, push it in hard and measure the clearance between the shim and the lobe base circle and record the
reading. Use a dial caliper to measure valve head (cleaned) to stem length as a datum for yourself to keep tabs on the machinist. Do this for all of
the valves, then take the valves back to the rebuilder with the calculated amount he needs to take off the stems, and have him do it. I forget the
exact values, but you want the exhausts at a mean value of something like .009", and the intakes at .005" (look at the TSRM to make sure). The
rebuilder can install the valves for you, but shim the springs just a little. I'm using 5/16" thin washers bored on the lathe, but companies like
McMaster-Carr (McMaster.com) sell all types of washers, some of which may be suitable as shims directly. Of course you could probably buy shims made for
the purpose.
I made up a tool here to use for valve spring installation with a large C clamp, just a piece of 3/4" tubing with windows cut in the side, a solid
end plug for the C clamp end and a plug with a 5/8" hole for the valve retainer end. With tweezers or needle nose pliers, you can get the locks in
and out of the retainers easily thorough the windows in the tubing.
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Hi. Just going through some informal analysis of 7MGTE internal oiling considerations, some good, some not so good. Good results indicate long lower end life if sump oil level is maintained. Not so good results emanate from an aparently poor cam oiling system which ultimately will wear at least the rearmost exhaust cam lobes to the point where a cold overnight startup knock will result.
1. The cams are hollow for oil distribution to each journal. Oil enters through cross drilled holes in the first journal from a .150" hole in the front aluminum brearing saddle for each cam. Each camshaft journal is the same size and each is cross drilled. There are 7 journals in total on
each camshaft. Entry holes in the first journal are about .115". although the front journals are deeply grooved for continuous oil supply to the hollow camshafts, ID of the internal camshaft passages is not presently known. The oil path in the head feeding the first journals is tortuous, in
turn fed by a .230" hole from the side stream hole in the block main gallery.
2. The act of centrifugal force pushing oil out of each camshaft journal through the lubrication holes indicates that the last journals receive less oil than the first. Indeed this was the case with a late JDM engine I've torn apart, where soft "mystery" metallic coating on camshaft bearing
journals was almost absent on the two rearmost exhaust journals. Nominally that coating is about .001" to .0015" thick. Clearance at the two rear exhaust journals was found to be .006", not far from the .005" allowed by the TSRM, a TERIFFICALLY high clearance for a small journal. So it
appears that the rearmost (at least) exhaust journals were almost starved for lubrication. That large clearance was probably responsible for the top end startup knock. Clearance was determined by removal of the buckets and shims, allowing lobe clearance, and use of Plastigage.
3. Before a 156K mile BHG on the original engine, an overnight top end startup knock indicated that something was amiss regarding consequences of inadequate head oiling or bearing design. Clearances on the JDM replacement engine confirmed this, so I'm starting to think that those of us who
have a top end startup knock can pretty much know what the noise is due to. I milled and filed the camshaft caps to get the Plastigage indiated clearances down to .003" maximum (mostly less), although filing is not a procedure applicable to rod and main bearings due to insert crush
considerations. I'm also thiking about opening up first journal oiling passage sizes in the head. The cam oiling is one consideration which does not sit well regarding acceptance of only a few PSI being sufficient at idle speed.
4. Increase in camshaft bearding clearances seems regenerative, in that the more wear occurs, the less oil will be provided to the rearmost journals due to lower pressure at those points.
5. The bottom end is built like the proverbial brick outhouse, with generous passage size, oil squirters, cross drilled main and rod journals, and rifle drilled connecting rods for additional wrist pin or cylinder wall oiling. None of my JDM rod or main bearing were out of spec, depite the
upper end oil starvation. Just run one auart high or deepen the sump and pickup.
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Hi guys. Thaks for the info. The camshafts were definitely coated with some very soft material before the lobes (free of coating) were ground. The coating is not a deposit from delayed oil change or anything of that sort. '90 cams and cams from a late JDM engine I'm rebuilding were the
same regarding the coating.
The photos tell all in good detail. I'm almost beginning to think that the plating is eutectic solder or perphaps a lower meling gallium alloy (a ternary alloy of gallium melts in hot water; spoons of that metal which melted in hot coffee were once sold as a novellty item), as it is softer
than babitt. Maybe Toyota had a problem with early startup camshaft journal oiling, and solved it by tinning or plating for jounal protection, since melting of eutectic solder occurs at about 370 F (or lower for the gallium alloys). It would serve as a short-term lubricant of sorts, although I
shudder to think of the outcome once oil (cold oil) were to make its way to the journals.
The coating is about .001" thick at the journals. I removed the cams from the JDM head, a task impossible to do without deforming the soft coating due to side thrust at the bearing cap parting lines from the open valves. It smooths out, however, when rubbed with a piece of precision ground
tool steel. Apparently it is softer than eutectic solder is, reasonable from a delta hardness standpoint since the saddles and caps are aluminum. It also smells soft, sort of a sulfurous odor. I'm half inclined to remove the stuff, and to reset the journal bearing clearance by filing or
milling the caps. No doubt I'm considering this out of ignorance, as Toyota had a reason for providing the coating in the first place. If it were removed, however, bearing clearance at the cap parting line would be .002" greater than is now the case. There is a school of thought that indicates
greater clearance in the axis normal to the load axis to be desirable from a standpoint of hydrodynamic wedge formation. This is customarily the way connecing rod bearings are sized.
Postscript: The coating was worn off of the pressure sides (opposite the lobes) of the last two journals of the JDM exhaust cam. Clearances were high at up to .006" (the TRSM allows .005"), an AWFUL lot for small journals, measured using plastigage after removing the buckets and shims for
lobe clearance. Since all of the wear was in the tops of the cam caps, I carefully milled and file "touched" them to get .003" maximum clearance, OK by the TSRM (that procedure is definitely not to be used for connecting rod bearings). Still need to get the JDM head and engine back together.
Additional postscript: The rebuilt JDM engine is going to replace a stock '90 turbo with a BHG at 156K miles. For the last 25K miles before the BHG I had a short overnight startup knock which always appeared to come from the top end. It was baffling since there are no hydraulic lifters.
Given the wear found in the JDM cam caps, the knock becomes understandable. Have used Mobil 1 since 45K miles.
