Distributor Lower Bushing Excessive Wear History
Gene,
I rebuilt this distributor last, in midsummer 2012, and have since put about 5000 miles on it. When I put it on the dyno this October, 2013, the torque/power curves showed very erratic, characteristic of an ignition problem. I didn't bother checking the timing before the dyno session. Of course, if I had, then a more accurate test could have been done. As it turned out, after checking timing at the shop, the WOT advance showed only 28 degrees (initial had moved from 21 to 13)! The dist had been set up with 36 WOT degrees when done in summer 2012, as it had moved 8 degrees retarded in 5K miles. The thing still made close to 400 RWHP @ 6650 RPM despite the loose distributor gear, retarded timing, and still not optimized a/f ratio. It also explains the mild tip-in stumble which has slowly been creeping in over the last few months or so.
The camshaft is a hardened ductile iron solid roller, which is used for non racing applications and so has a hardened iron distributor drive gear. It does NOT require a bronze or composite distributor driven gear, as billet steel camshafts do. The cam is holding up very well; I've checked lash three times now, in about 8K miles and it has not moved one iota. The cam is located fore-and-aft with a roller thrust plate to prevent rearward movement. Forward motion is limited by a steel roller thrust button up against the stamped steel timing cover which I reinforced by brazing a 1/8" steel plate to the inside. Fore and aft motion is limited @ .006", and this checks out; recommendation IIRC is max 0.010". Oil pump is standard volume with Melling pink spring (70#). Variation from stock is use of tool steel pump drive with integral coupling in place of the nylon coupling. I can't see if/how that setup can make a difference, but if I'm missing something then please let me know.
Gear mesh was checked and the pattern shows proper vertical distributor placement. I installed a new melonized steel GM distributor driven gear which is for use with ductile iron GM camshafts as well as my aftermarket ductile iron cam. I reamed the lower bushing for .004" clearance to the new shaft which I polished progressively (IN THE DIRECTION OF SHAFT ROTATION, so that the "grain" lays over in rotational direction) with 600/1000/1500/2000 grit paper. The shaft, as delivered from LICS was quite rough, and polishing removed about 0.002". The final pass with the reamer removed 0.002" more than I would have liked, so now, time will tell how well the wide .004" clearance will hold up. I drilled 2 - 1/32" diameter holes from the oil gallery access ring through the distributor housing and the lower bushing, so it will now be pressure oiled just like crank bearings. There is plenty of drain-back to prevent the housing filling with oil and flooding the Hall Effect trigger. This pressure feed also provides additional oil, in addition to normal splash, for gear lubrication.
I rebuilt this distributor last, in midsummer 2012, and have since put about 5000 miles on it. When I put it on the dyno this October, 2013, the torque/power curves showed very erratic, characteristic of an ignition problem. I didn't bother checking the timing before the dyno session. Of course, if I had, then a more accurate test could have been done. As it turned out, after checking timing at the shop, the WOT advance showed only 28 degrees (initial had moved from 21 to 13)! The dist had been set up with 36 WOT degrees when done in summer 2012, as it had moved 8 degrees retarded in 5K miles. The thing still made close to 400 RWHP @ 6650 RPM despite the loose distributor gear, retarded timing, and still not optimized a/f ratio. It also explains the mild tip-in stumble which has slowly been creeping in over the last few months or so.
The camshaft is a hardened ductile iron solid roller, which is used for non racing applications and so has a hardened iron distributor drive gear. It does NOT require a bronze or composite distributor driven gear, as billet steel camshafts do. The cam is holding up very well; I've checked lash three times now, in about 8K miles and it has not moved one iota. The cam is located fore-and-aft with a roller thrust plate to prevent rearward movement. Forward motion is limited by a steel roller thrust button up against the stamped steel timing cover which I reinforced by brazing a 1/8" steel plate to the inside. Fore and aft motion is limited @ .006", and this checks out; recommendation IIRC is max 0.010". Oil pump is standard volume with Melling pink spring (70#). Variation from stock is use of tool steel pump drive with integral coupling in place of the nylon coupling. I can't see if/how that setup can make a difference, but if I'm missing something then please let me know.
Gear mesh was checked and the pattern shows proper vertical distributor placement. I installed a new melonized steel GM distributor driven gear which is for use with ductile iron GM camshafts as well as my aftermarket ductile iron cam. I reamed the lower bushing for .004" clearance to the new shaft which I polished progressively (IN THE DIRECTION OF SHAFT ROTATION, so that the "grain" lays over in rotational direction) with 600/1000/1500/2000 grit paper. The shaft, as delivered from LICS was quite rough, and polishing removed about 0.002". The final pass with the reamer removed 0.002" more than I would have liked, so now, time will tell how well the wide .004" clearance will hold up. I drilled 2 - 1/32" diameter holes from the oil gallery access ring through the distributor housing and the lower bushing, so it will now be pressure oiled just like crank bearings. There is plenty of drain-back to prevent the housing filling with oil and flooding the Hall Effect trigger. This pressure feed also provides additional oil, in addition to normal splash, for gear lubrication.
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