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Old 05-30-2018, 10:25 AM   #1
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rebuild or replace engine?

Spun a rod bearing on a 2000 S while on the track last week. damage turned out to be rather minimal, limited to one crank journal and the single rod. Race engine builder told me crank would be repairable if Porsche sold undersized bearings, but since they don't appear to, I need a replacement crank.

So.... should I buy a used crank and rebuild motor, or should I find a used motor to drop in - both are about the same $ if I don't have to do any additional work to the used motor. My engine otherwise was in really good shape - no cylinder scoring, valves, pistons, heads all in great shape. Engine has Y in number, so it was a factory replacement engine. Odometer reads 99k, so figuring motor has between 40-60k on it. I'm leaning to rebuild since I will know everything about the motor. Anyone else out there done a rebuild with a used crank?

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Old 05-30-2018, 10:43 AM   #2
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REBUILD! You'll know exactly what you've got. While you are in there, hit some of the other known problem areas such as the rod bolts & IMS. Also, make a few updates. Replace the variocam pads & chain ramps. Replace the valve springs, rebuild the cam followers, and maybe get the heads ported. Get the injectors rebuilt. Add a sump that addresses oil starvation under high-g turns. You'll have a dependable engine that performs better than new. And much better than a used engine.
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Old 05-30-2018, 12:29 PM   #3
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I agree with j.fro make more sense, a used engine is a used engine. A rebuild is basically a new motor, as mentioned oppurtunity to address those know issues, maybe do some internal tweaks get some more ponies.
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Old 05-30-2018, 01:07 PM   #4
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Looking into this as well.
The challenge I see is that, if one rebuilds themselves, there is a chunk of the cost associated with special tools, manuals etc... to do it the correct way. Also, there is a learning curve vs a chevy V8 rebuild, for me anyways. I made a spreadsheet for minimum component replace (and no upgrades), and you're still looking at $2-$3K. If you upgrade springs, redo heads, valves, lifters, tensioners, chains, guides, upgrade IMS etc... just in parts alone it will be more like $4-$5K, not including any work to the crank case. I do like the fact that you know where you are though and that it is basically a new and better than new engine. With used, it's like a box of chocolates, you never know what you gona get BUT, you can also factor in some savings if you part out or sell the damaged engine.
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Old 05-30-2018, 01:57 PM   #5
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Originally Posted by AZ986S View Post
Looking into this as well.
The challenge I see is that, if one rebuilds themselves, there is a chunk of the cost associated with special tools, manuals etc... to do it the correct way. Also, there is a learning curve vs a chevy V8 rebuild, for me anyways. I made a spreadsheet for minimum component replace (and no upgrades), and you're still looking at $2-$3K. If you upgrade springs, redo heads, valves, lifters, tensioners, chains, guides, upgrade IMS etc... just in parts alone it will be more like $4-$5K, not including any work to the crank case. I do like the fact that you know where you are though and that it is basically a new and better than new engine. With used, it's like a box of chocolates, you never know what you gona get BUT, you can also factor in some savings if you part out or sell the damaged engine.
Good point, but sounds like he has an engine builder already assisting, and the engine torn down. Thing goes back to used unknown or spend cash and do it right. A used engine could spin a rod bearing or last years. Since he made the comment that it happened on a track day, sounds like its a car that will be track driven so would think it would be better to go the known route. These are basically expensive toys for most of us I think.
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Old 05-30-2018, 03:45 PM   #6
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I just ordered up first round of parts. Came in at $3k plus I am also installing the LNE spin on filter and 2 Qt deep sump which ran another $1k. The used crank was $900 (included in the $3k), I was told it was inspected and polished. comes with a 30 day warranty. Replacing all the timing chains/guides. Upgrading rod bolts to ARP bolts. Still have to inspect the heads, but engine was really good relative the the miles when I bought the car - which makes sense as it is a factory replacement. Did the LNE IMS this winter - 2k miles ago! I still need to remove the cover and inspect but it spins effortlessly. The debris was limited in the engine. Looks like the bearing overheated, seized, then stalled the engine without much time for any of the debris to travel. There was no scoring on my crank, just 1 burnt journal. Crossing my fingers I can keep the bearing - but we will inspect that this weekend.

I was considering building it to 3.6, but that would run an additional $7k minimum. My wife and I switch cars at the track, and I could almost pull the same lap times with her Miata that has coilovers and swaybars as I was with the Boxster. She needs ponies, I need suspension!
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Old 05-30-2018, 03:49 PM   #7
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If I might ask a question about re-building these engines. I was told that the cranks are reused and polished and run standard bearings . I have an engine that has low oil pressure at idle when motor is at operational temp.If I run the motor hard and come to a stop it will trigger a low oil pressure warning light while stopped till I drive again.Once idle come up light goes out.I did the oil pressure bypass update. And this did nothing for the problem. So the problem must be to much clearance between the bearings and crank journals.So the polishing of the stock crank will remove at least .0005 a half thou to a thou .001 so any rebuilt motor will have a low oil pressure a lot sooner than a motor with a new crank. Can the crank be welded and ground to stock dims.
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Old 05-31-2018, 04:50 AM   #8
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If I might ask a question about re-building these engines. I was told that the cranks are reused and polished and run standard bearings . I have an engine that has low oil pressure at idle when motor is at operational temp.If I run the motor hard and come to a stop it will trigger a low oil pressure warning light while stopped till I drive again.Once idle come up light goes out.I did the oil pressure bypass update. And this did nothing for the problem. So the problem must be to much clearance between the bearings and crank journals.So the polishing of the stock crank will remove at least .0005 a half thou to a thou .001 so any rebuilt motor will have a low oil pressure a lot sooner than a motor with a new crank. Can the crank be welded and ground to stock dims.
So are you saying you have a used or polished crank in your car? I tried to read up on what I could do to salvage my crank, and what I found was only a few people had luck welding and re-grinding, or plating and regrinding. At this point my thought is a good used crank that I was told was checked and polished was better option.

I just measured all my rod journals this morning with calipers (not perfect, but my micrometers are at work) to see if anything happened to the burnt journal, and all six measured exactly 52.98mm. I will measure both the original crank - main and rod journals, and the polished one I receive and post the results. I will then polish my original crank to see how much they change - plus I can see it becoming some form of furniture like an end table, so shiny journals will look better .
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Old 05-31-2018, 09:12 AM   #9
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From what I know my motors inside is stock with 125k miles on it. The question is how long before a crank that is polished will start to have oil pressure issues. I cant understand why no one makes bearings that allow you to grind of .010 to fix any problems , If you look at the surface were the bearing spun under a microscope you will see a very bad surface, It is actually peaks any valleys ,to get it polished removes all this .In the process it reduces the pin diameter.This will at some point have an effect on oil pressure. What is the tolerance on main and rod pin diameters?
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Old 05-31-2018, 09:15 AM   #10
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https://www.design911.com/Porsche/Boxster-986-987-981/Main-Bearing-Sets/pt856_865_-cma81-cmo82/ They have specially made oversized bearing shells. Also: https://www.europeanpartssolution.com/over-sized-bearings/
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Old 05-31-2018, 06:06 PM   #11
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Make sure you lock the timing to zero and install the tools to lock down the back end of the cams before removing the cam covers or the valve springs can break the cams when the cam cover is removed during teardown. Strange but true, this is no Chevy small block.

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