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Hmmm... this is a good point. Three points define a curve, but two points only define a line. So if the (two) bearing surfaces are parallel you don't actually care what shape banana it is in between....
They do spin at twice the speed of the crankshaft. Unless an experienced engine builder knew otherwise, I'd aim for tolerances similar to what is expected in a crankshaft. It should be easy enough to find good used ones though, for far less than what a shop would charge to fix your originals.
Thanks guys for the input. Highly appreciated. I might be in the market for another one, or two.. I want to pull the upper one before going further. If it's bent too, that may just mean that it was OK by Porsche QC. Perhaps someone on here has a way to access Porsche drawings and specs? But it seems strange to me, that if the shaft is bent, then the bearing surfaces, although good within themselves, would not be parallel, as the shaft would be in somewhat of an arc. This, IMHO, would produce a lot of bearing wear on the bearing surfaces, and would make it more prone to binding and increasing internal friction within the engine itself.. If this were the norm, it baffles me as to how Porsche would let this get out onto the production line, but, if they were produced by an outside vendor, then they would probably be checked sporadically by the QC dept, say 1 or 2 shafts per hundred and the defect would go unnoticed. Also, maybe these aren't the original shafts, or were damaged by a PO? Anyway, it's good to know. I'll keep you posted as to how things progress. Thanks.
What is the history on this engine? Are you doing a rebuild? Was the engine running ok before the rebuild? If you are doing a rebuild and the balance shafts were not a problem before the build I would think they are OK. If you changed the rear balance shaft bearings and started having the problem I would think the problem is with the bearings..
Be careful about thinking you've got an issue with balance shaft clearances. There's only one axial position (fore and aft) that will allow the balance shaft to spin easily. They will frighten you when spinning them if they're too far aft or forward. The "happy place", once you find it, is precisely where the shaft will be positioned once the proper hardware on the snout is in place. It's easy to think you have a problem when you don't. I've installed many, many balance shafts and have never had a problem. Of course make sure cover numbers match the block, and installed with the numbers facing up. I rarely see the snout bearing (the circular one) damaged. If it's visually okay, don't try to replace it and risk damaging the edges. A deburring tool will put a chamfer back on the edges if necessary, but re-using a serviceable old one will be fine.
My .02.
MM
Dear Charlie, Gage and others: Thank you for your thoughts and inputs on this matter. Apparently the bearing surfaces are parallel. That's the good news even if the shaft is a banana. Are there defining numbers on the BS covers that tell me that 1 is for the lower cover and 2? is for the upper? How do I know which is which? Perhaps I mixed them up, and if they are align bored they need to be put back into their proper place. I do know that they match the engine number. I did not know this about them being align bored when I reassembled the engine. As far as oil clearances go, a rule of thumb (unless specified) is 1 thousandths clearance per inch of diameter of the shaft . That works on cranks and cams as well.
The engine and head were gone through, and the engine machine shop owner and I, with the WSM in hand, rebuilt the total engine. The head was disassembled and rebuilt with a slight skim cut to insure against sealing and warpage problems. The valve springs were checked for height and strength.The guides were fine, so new guide seals were installed. Valve seats were kissed along with grinding and seating of the valves. All new engine seals throughout, along with new balance shaft bearings front and rear were installed. The clutch assembly was gone through, with the flywheel being resurfaced and a new disc installed. The pressure assembly was checked by a clutch rebuilder and was pronounced OK. New pilot bearing, release bearing, and clutch fork bearings and pin were installed. The engine assembly turned over easily when first rebuilt, but has now become problematic with the balance shafts being difficult to turn individually. The rotating assembly and camshaft continue to work/ rotate fine with acceptable effort. Thanks again for your ideas and inputs..I will continue to keep everyone posted as to progress? with this issue.
Balance shafts can bend ... ask me how i know !
Blew the cover off and locked up the engine at 94 mph going down towards turn 3 at Mont Tremblant, a rather
scary moment although it happened so fast.Gave me no prior indication before it blew.
Balance shafts can bend ... ask me how i know !
Blew the cover off and locked up the engine at 94 mph going down towards turn 3 at Mont Tremblant, a rather
scary moment although it happened so fast.Gave me no prior indication before it blew.
Cheers
Phil
Wow.. What would've been the cause of that? It's these kind of failures that give these engines a bad reputation.
I never figured out why the shaft bent like that ... the oil pressure was good and the bearings on the lower shaft were in good shape
although the upper ones were trashed at that point.Haven't seen any other posts here or on Pelican regarding this issue.I'm guessing the engine
didn't enjoy spending it's racing life mostly in the 6500 rpm range.
Another one of life's mysteries !
For the peanut gallery , i do use Loctite 574 when i build a new engine
OK guys, and a lot of people who know more about these engines than I probably ever will: I went to the rear of the engine block today, and here is the engine # 430-H197. Neither of the balance shaft covers have any stampings on the front of the cover other than the cast in part #. Both covers have the number 687 stamped or cast? into the side of the front of the cover. One cover has the number 1 on the top and the other has number 2 on it. These appear to be cast onto the cover, rather than stamped into it. I'll have to look at the covers to see if the cast in #'s, 687, are on the same side of the covers, so I can install them numbers facing upwards. One other thing I have noticed. Since the shaft lays in the cover, the bearing half supports it in the rear. The snout supports it in the front. If the snout is bolted to the cover, ( 1 bolt) there is little if any, wiggle room for the shaft to go anywhere. If tested at this point for ease of spinning, it should be the same way once it is placed into/onto the block and its bearings. Sounds like the torque and spin approach would be the best way for the installation to proceed, AFAIK. Comments please, thanks.
Last edited by Tiger03447; 04-13-2021 at 11:44 PM.
Reason: Spelling
The 687 numbers on the covers need to visible from above (very early engines are 3 digits, 4 digits on later engines). They should also match the block number, which you'll be able to see forward of the lower right bellhousing to block bolt, and the girdle number, which is inside the pan and won't be visible.
Photos here are from the FSM which you mentioned having in hand during the "rebuild"? The numbers on the bearing bridges (1 is right or lower and 2 is left or upper) should be readable while facing the front of the engine.
Let me know if you want me to physically check anything on the engine ...i currently have an 85 M44/07 engine on my stand that i'm stripping
down to the block for some machine work for what will be my next race engine.The balance shafts and covers are coming off next ...
04-11-2021 | 04:54 PM
