Align Boring the Crankcase, Help educate this amateur... Options

[stephenaki]

OK, I understand the reason WHY you align bore the case, got it. My question with the 914 case is bearing selection after it is done.

Now, rebuilding my MG engine and selecting bearings was easy, measure the journals and choose the size needed. The 914 case however has a set on the journal and a set on the case.

If I have to align bore the case is there a special consideration on bearing selection or do I just measure journals and get a bearing set that accounts for the difference based on balancing of the crank.

May be an obvious question to some but I am confused and want to make sure I rebuild the engine correctly and order the right parts.

[TheCabinetmaker]

OK, I understand the reason WHY you align bore the case, got it. My question with the 914 case is bearing selection after it is done.

The 914 case however has a set on the journal and a set on the case.

I don't understand this question. The motors I have assembled had only one set of main bearings and cam bearing. Bearings are available in oversized OD, and/or undersized ID. Someone PLEASE correct me if I read the q wrong.

[hcdmueller]

vsq914, I think you are right.

From what I can tell you either get different bearings for your turned down crankshaft or bearings for your alignbored case. I don't know if there are bearings that will accomadate both.

[Joe Ricard]

Chances are pretty good that you wil not have to get your case line bored.

Jake says he is seeing some but not all.

Type IV case is considerably more stout that type 1.

If you line bore you should also get the registeres decked or squared to the new plane of the crank shaft.

Cheaper to buy another case IMHO.

[Cap'n Krusty]

Main bearings come in combinations of (over)sizes. Std/std, std/.010", .010"/std, .010"/.010", etc. They may come with oversize thrust measurements, as well, but I'm not sure of that. Actually, probably not. Once you're beyond .020" on either side, the selection becomes almost non-existent. The Cap'n

[stephenaki]

I think my confusion is based on the fact that one set of bearings stays on the case and the others are on the actual crank. I believe the responses have answered my question and I will have the case checked for alignment once I get it completely cleaned up.

[dbgriffith75]

think my confusion is based on the fact that one set of bearings stays on the case and the others are on the actual crank. I believe the responses have answered my question and I will have the case checked for alignment once I get it completely cleaned up.

That doesn't sound right. There should only be one set of shell (halved/case) bearings for the crank and the cam. If one or both is undersized for the bore, you should just be able to measure the journals and buy corrected bearings.

Did you have the crank and/or cam ground down? Because that should be the only reason you need to buy oversized bearings, and the machine shop that did the work should be able to tell you what size bearings you need.

Pics would be useful if you can provide them to give us a better idea of what you're talking about.

[sww914]

There is only 1 set of main bearings, 1 set of rod bearings, and 1 set of cam bearings. When you order main bearings, you order bearings that fit your crank and case. If your case has not been align bored and your crank has not been ground down, you order standard bearings. These will be stamped on the back STD/STD. If your case has been align bored, you order bearings that are oversized on their OD. They will be marked +.25/STD or STD/+.25. I can't remember which comes first, the case measurement of the crank measurement. The +.25 is the new case dimension, the hole is now .25mm larger than stock, which is the same as +0.010".
If the case has been align bored twice or it was really misaligned it will be +.50. The germans mark them in metric and the americans mark them in inches.
If the case and crank have been machined then the bearing stamps will reflect this with 2 numbers, such as +.25/-.25.
It's easy to tell if the case of the crank have been machined previously by measuring the bores in the case and the throws on the crank. If they are standard, they will measure in whole numbers- ie 76mm, 45mm, 82mm, whatever. If they have been machined, they will measure larger for the bores and smaller for the throws in increments of .25mm. If your crank throw is 44.75mm or the bore is 76.25mm then each has been machined.
I don't know if any of the numbers that I listed are relevant, I just made them up as examples. The rod bearings are different. They don't change the OD of the bearing because they can machine down the flats of the rod end and cap to make the hole smaller and then bore it round again, maintaining the standard bore, so if the rod throws on the crank have been machined down, the bearings will be marked -.25 to accommodate the smaller crank dimension.
I'm sorry that I don't have a type 4 case, crank, or rods laying around to give you actual dimensions, but my stuff is running right now.

[stephenaki]

Thanks all, the crank that I ordered is balanced as are the rods, I don't remember the measurement I was given.

I recieved my pistons and cylinders yesterday, 94mm, and once I get my crank and rods in I will have everything balanced. I am hoping to have the case checked for alignment this week but may have to wait until the first week in August as all the Germans are getting ready to go on vacation and usually nothing is open!

Will update my other thread when I get things checked and ready for re-assembly. Always nice to have people smarter and more experienced provide input. (IMG:style_emoticons/default/beerchug.gif)

[Jake Raby]

It is IMPERATIVE that if the case is to be align bored that you have the main bearings BEFORE the boring process is done..

This is because the oversize bearings as well as the stock bearings no longer have the stock dimensions that work with the stock main bore dimensions dictated in the Bentley manual.

There are few people that align bore TIV cases correctly, most simply apply the same tunnel dimensions as stock and thats incorrect for today's bearings that can vary as much as .002 from stock.

Oversize OD bearings are available .020 on the OD and as much as .030 undersize on the crank. I do have a very limited supply of 1mm+ thrust cut bearings, but they are pure gold.

[dbgriffith75]

I do have a very limited supply of 1mm+ thrust cut bearings, but they are pure gold.

Really? Does gold last longer and resist wear better? I wouldn't think so for being such a soft metal. (IMG:style_emoticons/default/biggrin.gif)

[stephenaki]

It is IMPERATIVE that if the case is to be align bored that you have the main bearings BEFORE the boring process is done..

This is because the oversize bearings as well as the stock bearings no longer have the stock dimensions that work with the stock main bore dimensions dictated in the Bentley manual.

There are few people that align bore TIV cases correctly, most simply apply the same tunnel dimensions as stock and thats incorrect for today's bearings that can vary as much as .002 from stock.

Oversize OD bearings are available .020 on the OD and as much as .030 undersize on the crank. I do have a very limited supply of 1mm+ thrust cut bearings, but they are pure gold.

Thanks Jake, will measure and order the bearings once I get the crank in and, if the case needs align honing, will ensure they are available for the process.