I am planning on pulling my engine out on my carb'd 308 for a full engine gasket replacement. What else do you recommend be done while the engine is out and apart? How important is it to replace the Main Engine Bearing? Any help would be greatly appreciated.
It depends on how worn they are and how deep into things you are prepared to go. How is the oil pressure? It should be measured with a gauge of known accuracy (not the one on the dashboard) with the engine at full operating temperature. If the bearings are worn significantly it is possible that the crankshaft will need to be reground to an undersize. If the crankshaft needs to come out, then so do the pistons (which will at least need a set of rings and possibly need replacing) Certainly the heads will be off and should get at least new exhaust valves and a valve job (probably some new guides as well). The water pump gets replaced here and the oil pump needs a good check over at least, and so on and so forth. I'm not trying to scare you, just make sure you know what you're getting into. A 30+ year old engine will almost certainly need some things that you didn't expect.
Measure 'em ... I've been amazed how little wear I see on them and the rod bearings. When I first started playing with these engines I replaced them "just because" like a Chevy ... bearings for a Chevy are only $25/set though so I started measuring them. I've only seen "significant" wear on the front main in the direction the alternator/water pump belt loads ... now I'm basing this on what the manual calls out as "acceptable" ... others with way way more experience might think more than .002" clearance is nuts so I'd wait for some more experienced opinions ... especially on an alumium engine which expands so much. cheers
I agree with Lucky. Once upon a time I had this bright idea to rebuild and balance my 2 valve 308 motor for more power. So before the teardown I ran nitrous threw the motor until I ran out of jets! Boy is this motor strong. When I tore it down the mains were still well within spec.. So while the easy answer is if you tear it down replace everything, like Lucky says the wear here is often nil... so measure it.
I recall re-using main and rod bearings on a '85 Porsche 911 engine with 180k miles. Upon bottom end teardown we found that the bottom end did not need to be torn down at all due to how perfect everything looked and measured. Valve guides are sort of a weak spot on the 911 engines of this vintage so we replaced those and reassembled the engine and it ran as new. I am inclined to agree with the measure and replace is out of spec recommendation here. No need replacing them if they are still within spec and have no major irregularities. spec for wear limit per Ferrari workshop manual for rod bearings = .12mm (.004724 inches). standard diameter of a new rod bearing is: 47.129mm-47.142mm (1.85547-1.85598 inches) spec for wear limit per Ferrari workshop manual for main bearings = .15mm (.005905 inches).
Thanks for all you help guys. The engine comes out this week, so it will be interesting to see how the bottom end looks.
Be sure to post pics! Especially of the bearings plz. Interested in seeing how they held up over the years. I asked around a bit on this and it seems the only main bearing shell that is reported to almost always have a bit of wear is that of the frontmost bearing which mounts at the flywheel. I still have yet to speak with anyone who says the wear has been all that great though.
That is my experience too. All were fine, only the one on the left showed wear. (80 K miles) Image Unavailable, Please Login
Who knows what, and why this wear presents itself-what does it mean-and what is the remedy? Car's Owner: you may wish to wait till you hear the answers to this.....you do need to answer thew following however... PS-what does the "other side" look like, what are your dimensional charts like, and how is the thrust facings? How is the Main Galley? More, or less than .003" out? Rod shells?{new rod hardware is CRITICAL} CIAO!
Technically, there is no Main Engine Bearing. There are, however, several engine main bearings. If you're up for taking the bottom end apart and measuring up the rod and main bearings then that's good practice. Be sure you know how the measurements should be taken and what amout of excess "tolerance" is acceptable wear.
Replace the cam belt drive bearings and exhaust valves while you are there. New sump baffels would be a worthy expenditure at that time as well. The main bearings are the least of the concerns. Dave
