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Stupid question: Line boring 32V cam bearings

 
Old 02-14-2019, 02:37 AM
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skpyle
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Default Stupid question: Line boring 32V cam bearings

Hello Gentlemen,

I have a random, stupid question: Is it cost effective (relatively speaking) to line bore the cam bearings on a 32V 928 cylinder head?
I have been paying attention to the threads on how bad cylinder heads are getting in terms of deck erosion from coolant. Take away is great effort is being expended some times to save cylinder heads.
End of 2017, I bought a set of early S3 heads because the deck surfaces were not terribly eroded. Too bad I missed that the engine had suffered oil loss and run dry. The cam bearing surfaces look terrible. Would it be worth even thinking about line boring the cam bearings? I ask because the supply of S3 heads is very finite. I have had the heads for the Red Witch overhauled, but another set of heads is not necessarily a bad thing.
Or, am I full of fecal matter and these early S3 heads are door stops...

Thanks for your time and advice!
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Old 02-14-2019, 08:43 AM
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I would not spend the money on them unless it was absolutely necessary - use/save that money for something with a higher priority.
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Old 02-14-2019, 09:59 AM
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Hello Swaybar, thanks for replying!

I am not actually doing this. Sorry for not wording my question correctly. I should have asked something to the effect of, “Is it reasonable to think I could save these heads by line boring the cam bearings?”
I have the heads, but don’t need them. I already have a bitchin’ set of heads waiting to go back on the short block for the Red Witch. Those cost me enough. I am not spending money on these other heads. I was just curious if they are salvageable or bonafide door stops.

Thanks!
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Old 02-14-2019, 10:20 AM
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Originally Posted by skpyle View Post
Hello Swaybar, thanks for replying!

I am not actually doing this. Sorry for not wording my question correctly. I should have asked something to the effect of, “Is it reasonable to think I could save these heads by line boring the cam bearings?”
I have the heads, but don’t need them. I already have a bitchin’ set of heads waiting to go back on the short block for the Red Witch. Those cost me enough. I am not spending money on these other heads. I was just curious if they are salvageable or bonafide door stops.

Thanks!
save them they will be worth something at some point
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Old 02-14-2019, 11:37 AM
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Originally Posted by 928sg View Post
save them they will be worth something at some point
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Old 02-14-2019, 10:12 PM
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Makes sense. They are not eating anything and are not going anywhere. I have the heads wrapped in plastic in storage.

Thanks!
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Old 02-14-2019, 10:19 PM
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If you get "hurting" for room....strip the heads. Save the valves, since they are NLA and valuable. Save the lower spring seats, the shims, the retainers, the keepers, and the springs.

Recycle the aluminum head....it's junk.
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Old 02-15-2019, 12:29 AM
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S3 cylinder heads in question are here in this thread, starting at Post #42, going to Post #52.


https://rennlist.com/forums/928-foru...e-heads-3.html
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Old 02-15-2019, 11:31 AM
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Hmm, maybe someone needs to make the heads as well as the block as well.

As to your question of boring, most well equipped automotive machine shops that have Rottler cam boring machines can do them, but the question is what do you do with an overbore cam journal afterwards. There's no provision in the 928 world to accommodate this since the cams don't use bearing inserts. Now you're talking custom cams or bearing inserts.....not worth it on that motor.
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Old 02-15-2019, 02:46 PM
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Cut the caps and bore it back to the original size.
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Old 02-16-2019, 02:51 PM
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Catorce, thanks. However, I am a gearhead and not a machinist, so work with me. I should have said 'align boring' instead of 'line boring.' I am envisioning what is done when align boring main caps for the crankshaft on an engine. The faces of the caps are machined, then the caps are bolted and torqued down. The main bearing bores are no longer round. The boring bar then makes them perfectly round again. Granted, the centerline of the crankshaft is now moved X number of thousandths of an inch up into the crankcase. In the case of the S3 cylinder heads, I would think it would be possible to machine the cam bearings to the proper surface finish as they were. The slight movement of the cam shaft centerlines down into the head could be dealt with by the chain tensioners and the belt tensioner. I think.

Strosek Ultra, Thank You! That is pretty much what I had in mind.
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Old 02-17-2019, 10:13 AM
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Originally Posted by skpyle View Post
I would think it would be possible to machine the cam bearings to the proper surface finish as they were. The slight movement of the cam shaft centerlines down into the head could be dealt with by the chain tensioners and the belt tensioner. I think.
Yes. But, you will also need to look at the installed valve spring and valve stem heights.
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Old 02-17-2019, 10:49 AM
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Originally Posted by worf928 View Post

Yes. But, you will also need to look at the installed valve spring and valve stem heights.
Ah...sheet. Would this slight loss in distance not be accommodated by the hydraulic lifters?

Thanks for all of your advice on this issue that may never get taken care of. It is good to know, though!
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Old 02-17-2019, 12:34 PM
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Originally Posted by skpyle View Post
Ah...sheet. Would this slight loss in distance not be accommodated by the hydraulic lifters?
That’s the edge of the world for my knowledge on the subject. It’s not the cam you’re worried about. It’s the valve stem. The relationship between all the bits between valve seat and the cam lobe have tolerances. All I know further is that my local head guy had me double check his specs against the WSM and Tech Spec Books for spring height and installed tip height. Apparently, with a reworked head where valve seats and/or cam bearing surfaces have been messed with you can have issues with ‘lifter preload’ as well as the more obvious potential spring height and coil bind issues.

I’d never thought about it until my local head guy schooled me a bit.
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Old 02-17-2019, 12:45 PM
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That makes sense.
Thank you!

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