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Micha
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Hey all.
While complete engine rebuild, is it necessary to extract the rear main bearing holder?
Don't think it was ever done before by PO's.
I can see in drawings that there is a gasket beneath.
If so, how do I pull it out? Any special tool needed, or can I go with a regular arm puller?
It has what seems to be 4 aluminum shoulders.
Thank you.

  • rear_main_bearing_holder.jpg
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Michael Steinmann
R51/3 1952
Engine Nr. 529466

Daves79x
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Depends

It depends on how far you are going to clean the case. If you are vapor or bead blasting, you need to remove the carrier. There are specific procedures for removing it, you really do need the Barrington book - it describes in great detail everything you need to do.

Dave

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Dave

Micha
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Thank you Dave. Currently

Thank you Dave.
Currently don't have that book (budget matter...) and would like to here from members from their experience.

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

ahistand
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Rear carrier

The carrier is a very very slight interference fit into the case. It doesn’t take much force to push it out. I made an aluminum push block on the lathe that is slightly smaller than the ID of the carrier, and has a shoulder on it to catch the edge of the carrier. I simply use about a foot long block of wood on the backside of that and just tap it out. You can also push it out on a press if that suits you better. If you could find a large socket that’s the perfect size you could also use that to push it out. But, if you use anything steel you run the risk of distorting the edge of the carrier as you press or tap it out.

Daves79x
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One

Not to be snippy, but one wrong move is going to very likely be far more expensive than the cost of the Barrington book. A veritable drop in the bucket compared to the cost of the special tools and parts to to the job right.

Dave

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skyler.robbins
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I agree... I bought the book

I agree...
I bought the book to not only support Chris and Barbara, but it pretty much has everything you need from basic to advanced repairs.
I found it to be a great read when I'm just sitting around doing nothing.
Although I won't be doing a major rebuild on my motor/transmission/etc etc, I just appreciate that someone has taken the time to write a very detailed "Bible" on our machines.
With having less and less people able to do the repairs, thank goodness we have this book to refer to.

Rob

ahistand
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Bearing holder

Your bearing holder will likely need to be replaced if it has any appreciable wear on it. Your picture isn’t clear enough to tell definitively but if you can feel a lip with your fingernail where the rear edge of the bearing sat, then it should be replaced. The rear bearing can theoretically move axially back and forth very slightly and over time causes wear in the carrier. So putting a new bearing in an old worn carrier can potentially accelerate wear on that new bearing. I’ll have to look in the literature to be certain but I don’t think there’s a maximum ID wear limit listed for that carrier....I’m not near my books right now maybe someone else can say for certain. These carriers are a little less than $200 if bought in the US, quite a bit cheaper from certain European vintage parts suppliers. Most motors with significant mileage need new ones from my experience. Installing the new one you can simply line up the holes with the studs and tap it in with a soft faced hammer. Or, again, you can press it in with a press if that feels better for you.

Micha
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Thank you all

Thank you all Smile

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

Micha
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rear bearing holder

ahistand wrote:

Your bearing holder will likely need to be replaced if it has any appreciable wear on it. Your picture isn’t clear enough to tell definitively but if you can feel a lip with your fingernail where the rear edge of the bearing sat, then it should be replaced. The rear bearing can theoretically move axially back and forth very slightly and over time causes wear in the carrier. So putting a new bearing in an old worn carrier can potentially accelerate wear on that new bearing. I’ll have to look in the literature to be certain but I don’t think there’s a maximum ID wear limit listed for that carrier....I’m not near my books right now maybe someone else can say for certain. These carriers are a little less than $200 if bought in the US, quite a bit cheaper from certain European vintage parts suppliers. Most motors with significant mileage need new ones from my experience. Installing the new one you can simply line up the holes with the studs and tap it in with a soft faced hammer. Or, again, you can press it in with a press if that feels better for you.

ahistand wrote:

Your bearing holder will likely need to be replaced if it has any appreciable wear on it. Your picture isn’t clear enough to tell definitively but if you can feel a lip with your fingernail where the rear edge of the bearing sat, then it should be replaced. there is a fine groove that I can feel with my fingernail The rear bearing can theoretically move axially back and forth very slightly and over time causes wear in the carrier can it? it is held in place with lots of bolts. So putting a new bearing in an old worn carrier can potentially accelerate wear on that new bearing. I’ll have to look in the literature to be certain but I don’t think there’s a maximum ID wear limit listed for that carrier....I’m not near my books right now maybe someone else can say for certain. These carriers are a little less than $200 if bought in the US, quite a bit cheaper from certain European vintage parts suppliers. yes they are expensive Sad Most motors with significant mileage need new ones from my experience. Installing the new one you can simply line up the holes with the studs and tap it in with a soft faced hammer. Or, again, you can press it in with a press if that feels better for you.

I will again appreciate your thought here.
Thank you.

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

ahistand
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Bearing holder

Looking over the factory literature the only definitive spec on determining wear on the rear main bearing carrier is, it says the bearing in the carrier is a “slight pinch fit”. So this leaves plenty of room for debate on at what point the bearing carrier needs to be replaced. As I mentioned, and as it seems you’ve found, my gauge for replacement is when the carrier develops the lip which can be felt with your fingernail. Could the carrier still be adequate if worn past this point? Probably, but the lack of any real data to support this means that it’s somewhat of a crapshoot.

As far as what holds the crank in place, there are no bolts to hold it in. What locates the crank fore and aft inside the case is simply the interference fit of the two main bearings onto the crank, and then the interference fit (“slight pinch fit” according to BMW) of the bearings in their carriers. The crank is further tightened up and finally located by means of the steel timing gear as a press interference fit pulling the crank forward and effectively sandwiching the front main bearing between the gear, and the front slinger and spacer ring. In other words, when the crank is placed in the heated case (factory manuals call for 180 deg F.) and the timing gear is pressed in place, and all the bearing interference fits are to factory spec, in theory, the crank shouldn’t move at all. But I’m leaving out some details of fitting the crank properly so this doesn’t get too long, but if everything isn’t installed right, the bearings can be put in a “bind” and accelerated wear begins immediately after the expensive engine rebuild is complete. I’m not an engineer, and maybe someone with some theoretical mechanical design background can shed more light, but I’m assuming that due to the slight imperfect rocking moment of the power pulses from the connecting rods being offset, I’m thinking that this puts enough fore and aft forces over many miles and years of use to cause the crank bearings to begin loosening up slightly both in the case, and also on their crank journals, that the crank then moves ever so slightly as well as the bearings, were only talking about maybe a couple thousandths of an inch, and so over time the physical wear and damage occurs on both the carriers and the crank journals. Some evidence that this occurs, other than obvious wear found on these old engines, is that for the higher compression R50S and R69S engines a barrel roller bearing was used in the rear, presumably to allow for higher axial loading to account for this phenomena. Some feel that bearing is an outmoded design for this particular application, that’s certainly up for debate, the factory BMW engineers felt it was an appropriate solution, however that was nearly 70 years ago. There’s a lot to all this crank stuff some of it has much to do with feel and subjective assessment, and some is bound by simple factory tolerance specs.

Micha
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Wow. What an article! I

Wow. What an article!
I really appreciate the time and information.
I will learn what you have written here.
I already know that I will consider replacing this rear bearing with a barrel roller type.
Updates will follow.

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

ahistand
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Barrel bearing

The barrel roller bearing is quite expensive, and may not be worth the cost for the relatively low-stressed R51/3 engine. But that’s up to you to decide. Also, you will need yet another tool, a bearing aligner, to install the crank back into the case. It’s a very simple tool, no moving parts, which can be easily made on a lathe.

Daves79x
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Many

Many reputable engine builders don't even use the spherical rear bearing even in the R69-R69S engines it was designed for. A standard front and rear crank bearing is quite fine for an R51/R50.

Dave

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Dave

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