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scottsmi Member
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Just finished a rebuild that combined a 1973 head #2440 to a later engine block 3836 to get the newer rear main seal. After a short break in period the exhaust cam bearing on #1 froze up and melted the seal. Upon inspection it looks dry compared to the other bearings. I had to use the newer sump and oil pick up with the newer block but I am wondering if the old head was not compatible. Cant tell if the belt jumped but I think it did as one of the rubber teeth is chewed up. Oil pressure at start up was a constant 60 lbs and the engine maintained normal temp the whole time. Thoughts about what happened? |
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Esprit2 Member
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All 9XX 4-cyl heads are compatible with all 9XX 4-cyl blocks. There is nothing in the part designs that would have resulted in oil starvation due to mixing & matching parts. You can put a late 920 head on an early rope seal 907 block, and it will fit & function correctly. Oil passages in the head route pressurized oil from the block to the front cam journals. The front journal has a groove all the way around the center of the journal, and a hole drilled into the cam in the middle of the groove. Then the cam is gun-drilled front to back (ie, the cam is hollow). Oil is pumped to the #1 cam journal, is collected in the grove, fed through the cross-drilling into the hollow center of the camshaft. Then all aft cam journals are fed oil from the cam via cross-drillings at each journal. If all aft cam bearings had oil, then I have a hard time believing the front journal was oil starved. If #1 was oil starved, then none of the other bearings down stream from it would have gotten oil either. I think you need to take another look, and revise your failure analysis. *~*~*~* The cam's fore-n-aft position and end float are controlled by a thick spacer/ washer screwed to the back end of the cam shaft. See item 31 in the attached JPEG. With the shim/ washer properly installed, the cam's end float (freedom to move back and forth) is only 0.001 - 0.008 inch. Not much. Was that spacer/ washer present, or omitted? If it was not present, the cam could have moved forward far enough for the end face of the front journal to rub directly on the front seal... possibly melting some rubber. The bolt that retains the shim/ washer to the back end of the cam also plugs the far end of the gun-drilled oil passage through the cam. If the shim/ washer was missing, was the bolt also not present? If so, the cam's center oil passage would have been 'open', venting the cam's oil pressure. All journals' lubrication would have been compromised. Does any of that sound like what you can see in the autopsy? Regards, Tim Engel Attachment: JH - Cylinder Head.jpg (Downloaded 87 times) Last edited on 09-07-2018 04:11 am by Esprit2 |
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Esprit2 Member
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Here's a sketch of the oil's routing through the head. The joint between the head and the cam carriers is not shown. In real life, there are hollow dowel pins at the front of the head that pilot into the bottom of the cam carriers... one pin per cam carrier. They're hollow because they're also the oil passages that flow oil to the cam's front bearing journal. Regards, Tim Engel Attachment: 9XX Cyl Head - Oil Passage Drillings for Cam Bearing Lubrication 94kb.jpg (Downloaded 24 times) Last edited on 10-25-2018 01:47 am by Esprit2 |
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scottsmi Member
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Hi Tim, As always you are a great source of knowledge. As I began my investigation testing both cams for mobility and zeroing in on the exhaust cam front journal as frozen. I blew air through the various holes in the cam housing and understood the purpose of the hollow dowels. Interesting that air also passed through the passage that the bolt holding the water pump screws into. I thought about the fact that if that bolt was too long it could block oil flow, but didn't seem to be the case. (couldn't read the note about that on the diagram you sent due to distortion). I then progressed to the head itself and pulled the head to examine. I noticed that the head gasket was placed incorrectly and the oil passage from the oil pump was mostly blocked by the gasket itself. Not sure how either cam survived this but indeed there was some oil in both housings. I believe I read in an earlier post that there are not any cam bearings that can be replaced, so I am trying to find replacement cams and carriers. Not sure if I can use the intake cam still since it spins freely and appears in good condition. |
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Esprit2 Member
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Check with Lotusbits in the UK for used cam carriers and cams. http://www.lotusbits.com/ I'm not sure I understand how the head gasket blocked the oil passage (flipped over ?), but I don't need to understand as long as you're now aware of the problem and make certain no part of the oil passage is blocked in the future. Cam & Carrier Data (UN-PUBLISHED - Save it somewhere) Camshaft Bearing Journal OD Front #1 -- 49.746 - 49.759 mm (1.9585 - 1.9590 inch) #2 -- 49.492 - 49.505 mm (1.9485 - 1.9490 inch) #3 -- 49.238 - 49.251 mm (1.9385 - 1.9390 inch) #4 -- 48.984 - 48.997 mm (1.9285 - 1.9290 inch) #5 -- 44.412 - 44.425 mm (1.7485 - 1.7490 inch) Rear Cam Carrier Bore ID Front, #1 -- 49.835 - 49.860 mm (1.96201 - 1.96299 inch) #2 -- 49.555 - 49.581 mm (1.95098 - 1.95201 inch) #3 -- 49.301 - 49.327 mm (1.94098 - 1.94201 inch) #4 -- 49.047 - 49.073 mm (1.93098 - 1.93201 inch) #5 -- 44.475 - 44.500 mm (1.75098 - 1.75197 inch) Running Clearance All -- 0.076 - 0.114 mm (0.003 - 0.0045 inch) "Cam bores are to be straight, and all surfaces are to be parallel to the cam bores." *~*~*~*~* To your point about air leaking out of a water pump bolt hole... That shouldn't happen, but it does from time to time. They screwed-up, drilled too deep, and broke into the water jacket. Oops. When you install the water pump, apply thread sealant on the bolt threads. That's "sealant", NOT thread locker. If use thread locker, you'll have a fight on your hands getting the bolt back out next time. If you don't use anything, then that bolt will be an ongoing coolant leak. Regards, Tim Engel Last edited on 09-09-2018 01:54 am by Esprit2 |
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