One, the screw-bolts are a pain to tighten given the fact that they have a large phillips head top and are made of a soft steel which tends to cause fairly easy stripping of the screw head. Toyota's most likely reasoning for doing this is to prevent overtorquing of the screws. This is important on anything bolting into the aluminum head since you don't want to damage the threads on something that expensive to replace.
Two, the screw-bolts tend to loosen up and/or backout as the miles wear on. This is in part due to the rubber dampener/washer underneith giving up some preload as it hardens or compresses permanently and in part to the bolts simply backdriving due to engine vibration.
Since the head is made of aluminum, special considerations had to be made in the selection of the fastener material. Aluminum is notoriously reactive to non-similar metals and will quickly either corrode or be corroded when part of a galvanic pair (usually it's the corrodee). Also, I wanted to choose a metal resistant to atmospheric corrosion as the fasteners are exposed to high heat/cold and humidity. After consulting my college textbooks, various corrosion specialists and even the materials people at work, I came up with the best possible solution -- A4/316 Stainless (CRES). A4 (AISI 316) is Navy-certified for use with products in direct contact with seawater and well as used in off-shore drilling rigs. It's used in marinas as the axle material on aluminum pulleys which have 20 year warranties. It's in the same position on the galvanic chart as chrome, so should logically be fine in this application since Toyota chose Chrome plating as their method of corrosion inhibition. Since this passivated CRES bolts don't have a plating to wear off, this should be much more durable. Also, since a galvanic pair of CRES/Aluminum has the aluminum as the cathodic partner, the galvanic current will flow from the bolts to the head. Since the mass differential is so large, the current will be extremely small and virtually trouble free I believe (the chrome hasn't hurt it). Still, as a secondary measure of protection, I decided to have the bolts coated by my usual coating guys with a molybdenum disulfide (MOS2) dry-film lubricant. This should both provide galling protection as well as resisting thread damage by decreasing local friction.
For those without the option of coating the material, you can also use a little No-Ox compound on the threads as well, which is a goopy material used in mating aluminum wiring to copper in houses with an aluminum wiring infrastructure. This galvanic inhibitor provides one last wall of protection.
The tightening problem was overcome by getting the aforementioned A4 fasteners with a standard metric hex head. After doing some measurements of the stock screw/bolts and compensating for the greater thickness using the lockwashers, a 6x30mm bolt seemed to work fine. This bolt has a ten millimeter head and so will be just as common under the hood as all the other 10s. Other than the head, the thread parameters are the same as stock. A 1mm pitch is retained. This simple upgrade allows for easy and repeatable torquing of the cam covers with zero slippage.
The backdriving problem was solved by using internal tooth star washers which are used in any cases where vibration is the culprit of backdriving. The little teeth dig into the underside of the bolt as well as the stock washer/dampener and prevent loosening with anything but the torque which can be applied with a tool. Simply hand-based measurements confirmed this on Stuart's car.
The quantities needed for the car are (12) 6x30mm bolts and (12) internally toothed lock washers. We chose a fully threaded design instead of a partial thread as a margin of safety just in case the washers and stuff didn't cover all of the non-threaded region. Stuart had at least one of his holes completely stripped most probably due to overtorquing. Luckily, it was in an outer hole and was thus fixed using a 6x40mm bolt and a nut below. If this had happened on any of the internal, blind holes, the thread would have to have been retapped and Helicoiled (yuck).
These bolts were sourced from Brikksen, a metric stainless steel fastener supplier which happens to supply lots of marine quality parts. They are based in Dallas, TX. You cannot buy directly from them, but they are suppliers to many stores around the country. Brikksen isn't the only place to get bolts made to the aforementioned specifications, so check around at your local hardware, machine and fastener stores.
Thus far, testing has shown these bolts to last as intended. The stock screwbolts with stock washers were loose in mere days after Stuart put a new set on. The upgrades are still going strong with good preload and torque maintainence. The recommended torque on the original screwbolts according to the 1989 Service Manual is 22 in-lbs.. I think this value is this low since the screwbolts will most likely strip with anything higher. I'm thus going to torque to between 25 and 30 in-lbs at most. At last check, a strong factory worker can generate approximately 15 in-lbs with his hands, so if you feel comfortable with a simple nut driver, feel free to tighten by hand and not use a ratchet :-).