Newbie '73T 2.4 MFI Engine Rebuild

[Inkblot]

Quick status update: I've been carefully measuring the heck out of everything, to see if I can understand what the issue is. I'll post pics soon, but the short version is that I don't yet trust my measurements!

My bore gauge can only reach bearing journals 1-4 (with the #8 bearing in place for alignment), and my measurements are showing that the journals are...undersized? I'm regularly seeing bore diameters of 2.43xx, when stock should be 2.44094 - 2.44169. Not sure how that would happen, but my bore gauge technique is certainly suspect. If my measurements are correct, that would certainly explain the crank binding.

Anyway, I'm going to re-measure everything, and look into getting/borrowing a longer bore gauge so that I can get at the 3-7 bearing journals. Or, I'll attempt one of the "manual" case-alignment techniques, so that I can use my current bore gauge from the #8 side.

I'm also going to see if Ted (machinist) can give me a hand. That's probably my best option!

Jake

[ed mayo]

You're readings are to be expected with a mag case. The main bearing saddles become out of round, actually closing up horizontally. The original bearings slowly wore that way, now with new bearings they pinch the crank.

[Inkblot]

@Ed: thanks- I guess that makes sense. I just can't quite visualize how the heat cycles over time would cause the mag to expand that way, but I'm willing to accept that that's what happens! I always thought that when metal heats, it expands, and then when it cools, it contracts- but, if anything, it would contract more than it had expanded... at least, that's what happens in welding steel (granted, we're not welding steel here...).


Anyway, here are some pics of the measurements.

Starting with the bearings themselves, I used an old ball-end dial gauge on a nifty stand. I liked this setup, because it let me apply just enough hand-pressure to the gauge to get repeatable measurements without marring the "babbit" surface of the bearing too badly. I took measurements at 3 positions along the center line of each bearing half.










Here's a table of my measurements.




I couldn't find a direct spec for the bearing thickness, but if I'm reading the shop manual correctly and doing the proper math, then the spec should be 2.48 - 2.49mm. My measurements are all right in there, excluding a couple of on bearing #1 just over at 2.5mm.

In other words, I think my bearings look fine. Time to check the case bores.

Jake

[Inkblot]

Before getting to the case bores, I wanted to check the crank itself. I put another dial gauge on a flexible stand to confirm the "roundness" of the crank bearing journals, and to attempt to check for any wobble in the crank. Unfortunately, the only dial gauge that would fit my flexible stand reads in inches, so I'll have to keep converting back and forth to mm just to keep things interesting.

I put my crank on my wooden stand, set the gauge, and turn the crank. While measuring the bearing journals supported by my stand, I found no deviation in readings (the dial never moved off of zero), which indicate that the journals are nice and round (as expected). This doesn't tell me anything about the straightness of the crank, though.






With my stand supporting journals 1 and 6, I tried to measure any deflection in the shaft at the #8 position. The shop manual calls this "runout", and says to put journals 1 and 7 on v-blocks and to measure #4 and #8 with a max runout of 0.04mm. So, my test isn't perfect. Also, my setup isn't perfect; and I quickly found that I could influence the measurement by leaning on the crank, or when I'd apply the initial torque to turn over the crank. It was a fiddly setup, to say the least... anyway, I measured 0.0254mm runout on #8, in what was certainly an exercise in false precision.






Ok, on to the journals.


Jake

[Inkblot]

I cleaned off all of the remaining sealant residue and bolted my case halves back together, without the bearings, crank, etc. I kept the #8 bearing in place to keep the case aligned. Here's a pic using the bore gauge:




I was able to reach into the case far enough to measure the bores for bearings 1-4. For the uninitiated, to use a bore gauge, you set the zero to a known dimension (I used a micrometer), then add or subtract the bore gauge reading from that. I measured each bore at three positions, roughly corresponding with the locations I had previously measured on the bearings themselves.

Here's a table of my measurements. You can see my gauge readings, then the subtraction from the "zero" dimension to calculate the actual diameter.




A couple of things immediately stand out. First, my bores are not perfectly round. I couldn't find the spec for "roundness" per se, but the data sheet shows that the bores should be 62 - 62.019mm. I interpreted this to mean that different bores (e.g., #1 vs. # 4) should be within that spec, but maybe it also means that a singular bore could diverge from round by up to .019mm (?). In any case, I'm seeing variance of 0.1mm within a single bore.

Also, and most obvious, is that the diameters are too small. The min spec is 62mm, and I'm showing a max of 61.93. This would certainly explain why my crank was getting pinched.

As I previously posted, I'm still not confident in my measurements- but signs are increasingly pointing towards needing an align bore job. I had been working under the assumption that my case had a clean bill of health, but, you know what they say about assumptions... Hopefully I'll get a chance to visit with Ted next week to check it out.

Good times!

Jake

[Inkblot]

I really wasn't happy with my bore measurements, so I went back and did it again. This time I was extra careful about setting the zero of the bore gauge, and I think that made a difference. I was also more careful to get the bore gauge as close to the center line of the bore, consistently across the 4 bearing bores I could reach.




Here's my new table of measurements. They're much closer to spec than the previous set, but, still, only 2 of 15 values are actually within spec.




If the trend continues, all I have to do is keep measuring, and eventually everything will be in spec! Ha.


Jake

[Richey]

Has your case been line bored ? If not , more confirmation that a high milage mag case needs line boring . Otherwise , I'll be curious to know the problem .

[30westrob]

I had the same issue on a stock build on a 73 2.4. The first time I put the case to gather, with through bolts all to torque, the crank spun fine. During final torquing of the perimeter nuts, one of the flywheel end studs pulled.

So I pulled it apart. Repaired the one perimeter stud and reassembled using same bearings, etc. this time the crank was stiff.

After much measuring I had the case line honed, not bored, to spec. Engine has been running well since.

To this day I do not understand why the crank spun fine on the first closure, and not the second.

I relate to your frustration. Rob

[Inkblot]

Thanks guys.

@Richey: No, the case hasn’t been align bored. Early on in this adventure I was advised that I should do that, purely based on it being a high mileage mag case- I should have listened! My machinist gave my case the green light, so I thought I was in the clear. Maybe he missed something, or maybe I wasn’t asking the right questions at the time. Even with that, I should have measured everything to my own satisfaction much earlier in the process. Many lessons learned!

@Rob: Glad yours worked out.

It’s a setback, but for the best... I’ll get it worked out.

Jake


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[Neil Harvey]

Jake,

Here's your way forward.

Establish that the housings are under size but straight. They may have pinched in across the parting line. If the alignment is OK, then hone the bores to the correct size. This brings the sizes back to where you need them to be.

BUT!! Be careful here as the hone will follow the front and rear housing bores. The hone will not make the bores straight. The honing Mandell will follow the bores. If its off in any bore, the hone will remove stock only where it touches, which can make the bores even more out of round.

If the bores are out of alignment, your only course of action is to bore oversize or deck and bore back to std. You are looking at $$ for repair labor. Hopefully you can get by with a quick hone to bring the bores back to size.

[Jonny042]

Just at a glance, I'd say .05mm is WAY out of round. I think in inches (old habits and all my measuring tools are in inches), so I converted and find that .05mm is .0019". That's 2 thousands. In oil clearance terms that a mile.....

I'd say your best plan of action is to find a machine shop that specializes in 911's (Ollies comes to mind but I'm sure there are others, maybe within driving distance to you). And get them to fix up whatever needs fixing. Bore it and straighten it and get it 100%.

That'll give you a good foundation to work from and you can rest easy.

[Inkblot]

@Jonny: Thanks, well received.

@Neil: Makes sense. Quick question:

Originally Posted by Neil Harvey

Establish that the housings are under size but straight. They may have pinched in across the parting line. If the alignment is OK...

Do you have any suggestions on how to establish the alignment/straightness of the bores? I have access to standard measurement tools, but not a lot of specialty stuff... Seems like it'd be a bit tricky to establish a true center line from which to measure the bores against, unless you mount the case into a mill or lathe or something...

Thanks as always!

Jake

[Neil Harvey]

Originally Posted by Inkblot

@Jonny: Thanks, well received.

@Neil: Makes sense. Quick question:



Do you have any suggestions on how to establish the alignment/straightness of the bores? I have access to standard measurement tools, but not a lot of specialty stuff... Seems like it'd be a bit tricky to establish a true center line from which to measure the bores against, unless you mount the case into a mill or lathe or something...

Thanks as always!

Jake

Yes, easier said than done for the DIY.

We attach a 1/10th indicator to our boring bar and as the boring bar travels through the case it will show any misalignment.
If they are way out, a simple straight edge will show up any misalignment. But if they are close, this will be too inaccurate.

Do yo have access to a std knee type milling machine with a digital readout in the "Y" direction?

Set a case half up on the table and indicate in the rear #1 & #8 bores and the case level with the table. Lock the "Z" so you measure the same height in the housing bores. Zero Out either end and then traverse along the "X" axis and measure each of the other housing bores. You can swing the indicator 180° to measure the other side of the housing bore.

Good luck.

[47silver]

https://www.porscherepair.us/porsche-911-guide/measuring-and-reconditioning-crankcase.html

[Inkblot]

Quick update: My case is now back with Ted @ German Precision. Turns out he's moved his operation out to Yuba City, and says he's as busy as ever.

I'll continue to poke around at a few things while I await the return of the case.


-Jake

[Inkblot]

Ok, back in action. Here's a few pics of my freshly align-bored case; thanks, Ted!







Just gotta clean 'em up, and then put it all back together (hopefully without repeating the mistakes of the past!).



Jake

[47silver]

Was it align bored or honed?

[Inkblot]

Bored, to .25mm over. Gotta get some new bearings now...


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[cmcfaul]

oversized bearings are crazy expensive if still available.

Chris
73E

[E Sully]

I've been enjoying this thread and am looking forward to the next update. Hoping it turns out well.

Originally Posted by Inkblot

Hi Chris, yeah- pretty sure the screen is normal/stock for my motor. I did have the little tabs for the oil pump screws, but I removed them in favor of Loctite (and as suggested by Neil Harvey).

Jake

I've read a few thread regarding the oil screens. According to the PET it ended with the -71 engine, but seems in real life they continued till the 2.4 7R case.
901 101 035 01 left: oil strainer -71
901 101 035 01 right
A few threads
https://www.early911sregistry.org/forums/showthread.php?116882-Do-you-need-the-oil-strainers-inside-the-crankcase&highlight=901+101+035+oil+strainer

Windage screens