It crossed my mind recently, so I've been meaning to understand the mentality of not using bearings in the head of an OHC engine. Since I'm notably weak on rebuilding know-how, perhaps someone can enlighten me on this. What happens if, say, one of the 'journals' the cam rides in was worn un-evenly and neeved to be corrected? Do they just shave, or trim where the cam rides, down across the board? Does the stock shim spacing serve as the adjustment for doing this? I imagine there's some limit to how much this adj. can be done. Do machinist build up and re-grind on cams, like they do cranks?
-Jamie
Why the lack of bearings?
-
deviousKA
- Posts: 1355
- Joined: Tue Aug 12, 2003 5:04 pm
- Car: 90 240sx NA /72 Datsun 510 NA /86 corolla GTS NA
- Contact:
This isnt usually a problem (wear) unless a low oil or debris condition. The camshafts are actually a very low wear item (the journals). Some engines use as little as 3 cam journals (like some toyota engines ie. 22r), These are more prone to failure and need to be repaired more often.
What some rebuilder shops will actually do, is enlarge the diameter of the jounal (by pressing collar over existing), they then line bore the head to an oversize. This is poor practice, they are just trying to save bad head and cam cores.
General practice for rebuilders is to use find the best used ones and stay with stock spec. Any welding/adding then grinding material off of a camshaft generally is to expensive, and getting another cam is a better option.
If the bearings in the head are worn/galled etc.. depending on severity they can be reused. Considering the amount of bearing surface area for cams.
What some rebuilder shops will actually do, is enlarge the diameter of the jounal (by pressing collar over existing), they then line bore the head to an oversize. This is poor practice, they are just trying to save bad head and cam cores.
General practice for rebuilders is to use find the best used ones and stay with stock spec. Any welding/adding then grinding material off of a camshaft generally is to expensive, and getting another cam is a better option.
If the bearings in the head are worn/galled etc.. depending on severity they can be reused. Considering the amount of bearing surface area for cams.
devious: Thanks for the response - good info there. Now that I think about it, there really isn't much lateral force put onto the cam journals (just enough to support the cam compressing the springs).
One reason I ask is because I have a journal, the front-most exhaust, that has a pit in it. Almost looks like a mfgr imperfection, like a bubble in the cast or something. Anyways, it's about 3-4mm in diameter and about 3mm deep. There's an associated wear 'ring' on the cam also. Also worth noting is a chunk, probably 1/4" wide, and maybe 1/5" deep out of the corner of one of the cam ends(away from home, unsure which). Not sure how it happened, I rx'd it this way(possibly dropped). My father says that they(cams) are usually hollow. Do I have reason for concern here? Wish I had a digi-cam at home, it'd come in handy about now. I have another set of cams from my 92(same grind as these 91s), however the engine threw a rod and I am leary about re-using parts from it.
Regards,-Jamie
One reason I ask is because I have a journal, the front-most exhaust, that has a pit in it. Almost looks like a mfgr imperfection, like a bubble in the cast or something. Anyways, it's about 3-4mm in diameter and about 3mm deep. There's an associated wear 'ring' on the cam also. Also worth noting is a chunk, probably 1/4" wide, and maybe 1/5" deep out of the corner of one of the cam ends(away from home, unsure which). Not sure how it happened, I rx'd it this way(possibly dropped). My father says that they(cams) are usually hollow. Do I have reason for concern here? Wish I had a digi-cam at home, it'd come in handy about now. I have another set of cams from my 92(same grind as these 91s), however the engine threw a rod and I am leary about re-using parts from it.
Regards,-Jamie