It’s been a while but we're back with more info on the engine buildup. And, since the lower end is pretty much done, we're starting to look at cylinder head reconditioning. Matt may have addressed some of this over time and I questioned the P90a to P90 conversion but bear with us...
Matt wants to use either the P90 or P90a Cylinder head because they are said to have better flow and combustion chamber characteristics. He and I looked all over for a head we could use, buying more than one, only to send at least two back to the sellers. I thought a warped head or one that had been surfaced too many times would be the big head ache but the biggest problem turned out to be corrosion.
Five heads and two engines later, when our buying frenzy was over, we ended up with one P90 and two P90a heads to choose from. None of the heads were warped but one P90a has been surfaced and needs 0.015 shims under the cam towers; the P90 has some minor issues with intake/exhaust manifold gasket surfaces (minor corrosion); and the second P90a has some pretty bad corrosion around a few coolant passages. All three heads need valve work, including new valve guides but all can be reconditoned. Otherwise, teardown showed that every rocker arm was shot and the hydraulic cams were badly pitted.
Matt bought new OEM rocker arms and I sent all three cams to Iskendarian for evaluation. Isky had the cams for about four months and I had to call on their status at least five times. When I finally got hold of Ron Iskendarian he appologized, had the P90 cam polished, and then shipped all three back without charging us a dime. I'd say he did us right.
Because the head gasket surface on our second P90a is the truest, and because our machinist tells us he can clean the surface with a 0.003-inch cut after welding up the corrosion, we decided to repair that P90a. But, like almost everyone else rebuilding an engine, we're limited to mechanical cams and need to make the modification that fits the smaller diameter mechanical pivot posts.
The mod is well known (except to me, before I started this project), and it's documented on the internet in more than just a few places. But while researching I came up with a few questions, such as: How to control pressurized oil seepage from under the pivot post; supporting the upper part of the Time-serts; and the mileage-life expectancy related to both those questions.
The question of oil seepage is easy to fix with short pieces of aluminum dowel in each oil gallery, inside the pivot post bores. The plan is to turn the dowel material to the same outside diameter as the inside diameter of the oil gallery bores; freeze the dowels for an hour or so; warm up the cylinder head to about 120 degrees in momma's oven (I didn't say that) and then tap the cold dowels into each gallery of the warm cylinder head. When temperatures equalize the interference fit should keep everything in place. (When I get to that point, I'll post the photos and narative.)
For the same reason that the pivot posts need oil pressure, about ½-inch of depth within the pivot post bore is larger in diameter than the threaded area below. That makes installing threaded inserts easier but leaves most of the insert without mechanical support.
Comparing a P90 head with a P90a (below left and right, respectively) there is about 50% less surface area to support a mechanical pivot post if installed in the P90a.
Also, using the threaded insert in the P90a only provides about three threads – just 2.25mm or so – where the head material supports both the Time-sert and the mechanical pivot post.
Time-serts used to modify the head are about 1/10-inch smaller in outside diameter than the pivot post bore and their installed height must be below the face of the boss. With a higher lift cam, the sweeping action of the cam lobe against the rocker arm is bound to increase lateral forces against the Time-sert. To eliminate the void we need either a machined sleeve around the threaded insert, or something lke an epoxy to fill the gap. Matthew and I decided to use machined aluminum sleeves but the cost would be enormous...
...So,I made about sixteen sleeves in my garage, using not much more than my drill press, my calipers, some files, and sand paper.
Here's an installed sleeve in a P90a pivot post bore.
Twelve sleeves cost me about $35 in materials and about 18-hours of time (because I didn't have a lathe). Not bad, I think.
Method to follow, later.
Cheers.