PD Precision Cam Bore Alignment
Frank, the OEM bolts have a wider area on them which i assume helps to locate the rockers in the correct spot. When reinstalling the rockers, i generally spin the OEM bolts back in snug, then remove and replace them one at a time with the non stretch ones snugged, THEN torque to spec.
Any thoughts on those wider sections of the OEM bolts?
Justin,
The OEM TTY cam and rocker bolts are not as accurate as some might hope. The barrel on the TTY bolts is supposed to center the cam cap on the head, but in many caps, we find the OEM cam bolt is loose in the cam cap's bore as much as .005". Our intention when aligning the cam bearings is to have the cam caps indexing closer to .0005", or ten times as close.
With those close tolerances, we often found the cam was binding in the cam journals. Obviously, additional steps were necessary in order to align the bearings in their caps, even if using the OEM bolts. We use a plastic mallet to strike the cam caps and jar the cam bearings into position and a three-stage torque method. The OEM bolts did little to align the bearings. We haven't used the TTY bolts for the cam or injector shaft for years. We got the engineering worked out, but the methods require some finesse and determination.
More recently, we have worked out two fixes for the cam bore alignment.
First, we have found a roll pin that will accurately index the cam caps to the cylinder head. The pin is pushed into the cylinder head and the cam cap fits on top of it. The repeatability of the hole alignment between the cam cap and the cylinder head is excellent.
Second, we found a cure for the unusual wear pattern in the #1 and #5 cam bearings. It is due to the torque load from the injector rocker shaft.
The PD cylinder heads were machined, initially, with the cam caps separated from the main casting. Then, the cam caps were attached and the cam was align bored. Unfortunately, there are two faults with the method.
1. The indexing is comparatively inaccurate (which we have already addressed...)
2. The injector rocker shaft's torque load was not considered.
The real force holding the cam caps in place is provided by the injector rocker bolts. But the clamping force is unequal for the #1 and the #5 caps, as there is only one offset bolt in the #1 and #5 cam caps. The #2, #3 and #4 rocker bolts are paired and provide equal force for both sides of the cam cap. But even then, there is some amount of squish which was not calculated in the manufacture of the head. The force of the rocker shaft bolts is pushing the cam bearings out-of-round AND out-of-square.
Our correction method is to install the injector rocker shaft in the same manner of a torque plate. Just as boring a performance block,
the purpose is to intentionally apply the the loads that warp the cylinder head and machine the head in that condition. We feel that this correction has cured several issues with cam bearing wear and alignment.
Initial evaluation is excellent. The material removed is precisely where the interference wear is occurring.
We will be using this method of cam align bore correction for each PD cylinder head we see in our shop. As for fixing align bores while the head is still attached to the engine, it is not feasible at this time.