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| main bearing help https://www.slantsix.org/forum/viewtopic.php?t=65301 |
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| Author: | dpstark2 [ Tue Sep 08, 2020 9:27 am ] |
| Post subject: | main bearing help |
I'm posting this on behalf of a friend who is building the /6 for his '68 Dart. He is an experienced engine builder (but not a pro). I've been through a couple engine builds with him and trust his judgement. The motor that came out of the car supposedly had 85k miles on it and was running, but in need of service. Since he drives his cars a lot, he decided to do a full rebuild on the motor. During disassembly, he noticed a "tight spot" while spinning the crank. This led to a line hone on the block, checking the crank, etc. All looked good according to the shop. Upon reassembly, the tight spot was still there. Eventually he tracked this down to bearing #3. When removed, the crank spins freely. Measuring where the bearings are installed, he found that each bearing bore was in spec, but maybe a bit less precise than desired. Suspecting a bent crank, the block and crank went back to the machine shop. The shop said everything was in spec, crank was straight to .0001-.0002". To try to understand the issue a bit better, he put gear marking compound on the bearing surface and spun the crank. The "wiping" of the compound was consistent for bearings 1, 2, and 4. #3 shows much less compound left on one half where the crank seems to touch or almost touch the bearing, and much more on one side where it must have a lot of clearance. I suggested finding under/oversized bearings and mix and matching. While that doesn't make me "happy," I can't think of any other ideas. Anyone have any better ideas? Or know where to get under/oversize bearings - because I don't. Thanks! |
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| Author: | Joshie225 [ Tue Sep 08, 2020 11:28 am ] |
| Post subject: | Re: main bearing help |
This is unusual. If a bearing is out-of-line the drag should be consistent, but seeing as it's a tight spot the crank is naturally suspect. Do you have a dial indicator so you can check #3 main journal for runnout while it's in the block? .001 and .002" undersize bearings used to be common, but those are only for a somewhat worn standard size crank you're trying to work with. You might try ebay for those. But really, if the main line was corrected and the crank is straight (hard for me to believe) you won't have a tight spot. There are tolerances for everything, but this engine is off somewhere. |
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| Author: | hyper_pak [ Tue Sep 08, 2020 5:14 pm ] |
| Post subject: | Re: main bearing help |
Do you have another crank to check in your block? |
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| Author: | emsvitil [ Tue Sep 08, 2020 5:48 pm ] |
| Post subject: | Re: main bearing help |
Is the #3 bearing shell only for #3? Can you move the shells around? |
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| Author: | ProCycle [ Tue Sep 08, 2020 5:51 pm ] |
| Post subject: | Re: main bearing help |
Seems to me for there to be a tight spot BOTH the block and the crank have to be less than perfectly straight. |
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| Author: | Joshie225 [ Tue Sep 08, 2020 6:09 pm ] |
| Post subject: | Re: main bearing help |
Quote: Is the #3 bearing shell only for #3?
Yes, that's the thrust bearing.
Can you move the shells around? |
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| Author: | emsvitil [ Tue Sep 08, 2020 7:08 pm ] |
| Post subject: | Re: main bearing help |
So maybe the drag has something to do with the thrust surface (bearing and/or crank) |
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| Author: | ProCycle [ Tue Sep 08, 2020 7:39 pm ] |
| Post subject: | Re: main bearing help |
Quote: So maybe the drag has something to do with the thrust surface (bearing and/or crank)
That makes more sense.
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| Author: | Joshie225 [ Tue Sep 08, 2020 9:53 pm ] |
| Post subject: | Re: main bearing help |
Quote: Seems to me for there to be a tight spot BOTH the block and the crank have to be less than perfectly straight.
If the crank is bent enough the mains can be perfect and the crank won't turn.
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| Author: | ProCycle [ Wed Sep 09, 2020 8:33 am ] |
| Post subject: | Re: main bearing help |
Quote: Quote: Seems to me for there to be a tight spot BOTH the block and the crank have to be less than perfectly straight.
If the crank is bent enough the mains can be perfect and the crank won't turn. |
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| Author: | Joshie225 [ Wed Sep 09, 2020 10:47 am ] |
| Post subject: | Re: main bearing help |
Right, I can visualize it now. |
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| Author: | dpstark2 [ Thu Sep 10, 2020 9:24 am ] |
| Post subject: | Re: main bearing help |
Thank you for all the thoughts. I agree this one is puzzling. I'm no engine expert, but obviously I would suspect the crank. However, there doesn't seem to be any serious run out on the journal. It's like the bearing bore is both out of place and maybe out of round, which is seems unlikely given that the block was line honed. I think the next thing he's going to do is get another machine shop to take a look. If it gets resolved, I'll post a reply. |
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| Author: | dpstark2 [ Wed Sep 30, 2020 11:35 am ] |
| Post subject: | Re: main bearing help |
Well the verdict is in from a second machine shop... this one cleaned up the crank and did a line hone and everything worked out. I guess you just have to find a shop you trust. Lesson learned. |
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| Author: | Joshie225 [ Wed Sep 30, 2020 4:22 pm ] |
| Post subject: | Re: main bearing help |
Yeah, I hear you. Glad it worked out and thanks for following up. |
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| Author: | volaredon [ Wed Sep 30, 2020 6:08 pm ] |
| Post subject: | Re: main bearing help |
so it was "line honed" 2x? I know they graze the main caps to make the bore slightly (emphasize "slightly") oval then line hone/ line bore them back to a true circle.... I'm wondering how much material was removed each time and how far a block could go before things like bearing crush would be a factor? Without enough crush, even with the locator tang on the bearing shells, that journal could be more subject to possibly having a bearing "spin" on the journal.... and 2nd question... these aint exactly vertical shaft lawn mowers hitting a culvert/ how would a car/truck crank get bent? |
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