TheSamba.com :: View topic - Main bearing

Wed Nov 26, 2014 1:16 am

Hello

We finally decided to make a rebuild of engine. We opened it and determined which parts need to be replaced, but here's a tricky thing, we need a main bearing set. It is "0.25mm", BUT there are some other characteristics listed on websites, for example:

Option 1: .25MM CRANK / .50MM CASE / 1.00MM THRUST
Option 2: .25MM CRANK / 2.00MM CASE / 2.00MM THRUST

Can anyone help me with English? What are these "case" and "thrust" values? I don't want to make a wrong purchase.

P.S. Engine is 1970-73 Bay window bus 1600cc.

Thanks in advance

Wed Nov 26, 2014 4:48 am

When you rebuild an engine there are three things to setup on the crankshaft, case, and bearings

If you grind the crankshaft to remove wear, it ends up thinner than factory. In this case it needs a fatter bearing to make up for the thinner crank shaft.

In your post, the bearings have been designed to make for a crankshaft that has been ground 0.25mm thinner from stock.

Likewise , if the journals where the bearing s sit have been pounded out by miles, you bore these out to make them round again, and....you guessed it, fit an oversize bearing on the outside to compensate.

Finally, the end play of the crankshaft is controlled at one end by the insertion of shims between the flywheel and the engine case. I'm not as sure on the spec on this one, but I think this is telling you the bearing set has a certain thickness to make up most of the end play gap and make it easier to shim to spec.

This is pretty serious bottom end engine stuff which needs to be right, and not just close. Work with a machine shop to get this right and buy them a six pack when you are done.

Wed Nov 26, 2014 5:45 am

Abscate wrote: When you rebuild an engine there are three things to setup on the crankshaft, case, and bearings

If you grind the crankshaft to remove wear, it ends up thinner than factory. In this case it needs a fatter bearing to make up for the thinner crank shaft.

In your post, the bearings have been designed to make for a crankshaft that has been ground 0.25mm thinner from stock.

Likewise , if the journals where the bearing s sit have been pounded out by miles, you bore these out to make them round again, and....you guessed it, fit an oversize bearing on the outside to compensate.

Finally, the end play of the crankshaft is controlled at one end by the insertion of shims between the flywheel and the engine case. I'm not as sure on the spec on this one, but I think this is telling you the bearing set has a certain thickness to make up most of the end play gap and make it easier to shim to spec.

This is pretty serious bottom end engine stuff which needs to be right, and not just close. Work with a machine shop to get this right and buy them a six pack when you are done.

Much obliged!

So, please let me summarize. If I understand correctly, "crank" value means crankshaft's trimming, therefore bearing thicker on crankshaft side (inside). "case" value means trimming of case, therefore bearing thicker on case side (outside), while "thrust" means end play adjustment, therefore bearing is wider, but exact value of that is determined with help of "flywheel shims".

Am I right?

Wed Nov 26, 2014 7:13 am

Yes.
It will take a bit more than a six pack for the machine shop to make everything right.

Wed Nov 26, 2014 7:41 am

my59 wrote: Yes.
It will take a bit more than a six pack for the machine shop to make everything right.

I figured that part already, about the six pack. Luckily crankshaft and case are okay, the only problem is end play. Then I'll be needing push rod tubes, cylinder shims and the least pleasant - that huge flywheel, shipping of which is going to cost a fortune. But on brighter side, I'll have the intact engine.

Thanks for the assistance!

Wed Nov 26, 2014 8:49 am

Harbinger wrote: while "thrust" means end play adjustment, therefore bearing is wider, but exact value of that is determined with help of "flywheel shims".

Am I right?

The front bearing area of the case the bearing is seated over it with a "U" cross of the bearing. As you shift the pressure of the clutch shoves back against #1 bearing, which in turn over time pounds that part of the case thinner. So at the machine shop the part of the case will be cut thinner to make straight/flat areas, which will require a bearing that narrower to fit snugly on the case.

Wed Nov 26, 2014 8:58 am

When I did my type 4, I measured the end play prior to dissassembly and then measured the shims once apart to see what I had.

The shop said not to order shims until the new bearings were in, as they were thinking that the existing shim stack would do the job with a new thrust bearing because the case measured in spec when they checked it, and the new thrust bearing thickness would take up most of the existing end play (IE the real issue with the engine was simply worn out bearings, not worn out case surfaces)
They were right.

Wed Nov 26, 2014 9:09 am

#1 thrust bearing at far right of image below:

Wed Nov 26, 2014 10:26 am

I know a good VW specific machine shop is something that is becoming harder to find these days. But any machine shop worth a damn that I have ever dealt with was able to provide me with the proper crank and cam bearing sets, matched to the machine work they did. This sort of thing would be a common service provided IMO.

Wed Nov 26, 2014 11:23 am

There are many factors that influence what bearings are to be used.

The proper way to do the job is:

1) clean all the parts
2) visually inspect them including looking for cracks
3) visually inspect the rods and rod bolts for fracturing
4) Feel the inside of the cylinder to see how deep the wear is at the top
5) If it is acceptable measure the cylinder bore with a bore gauge
6) Visually inspect the pistons for scuffing, pitting, cracks.
7) measure the pistons at the skirts to see if how much they have collapsed. If it is within acceptable limits they can be re-ringed and used. if not replace pistons and liners.
8)inspect the case mating to make sure the webs are parallel. Some folks use a feeler gauge, others use thin carbon paper. If the bearing wear was acceptable checking this may be OCD. If a main bearing spun or the engine overheated it is mandatory.
9) Bolt the rod caps on and check them for size. Resize if needed. If there are any shiny spots on the insides of the rods or the outside of the rod bearing shell it means the bearing is loose and has been working. Rod must then be resized.
10) check piston pin fit and replace and hone small end bearings if needed
11) Check main saddles on case for wear. If you can feel indentations with your fingernail it probably means it is time for an align bore. T1 cases are prone to this, T4 cases are not.
12) Use a micrometer to inspect all the rod journals for size. Visually inspect crank for wear. If size and wear is acceptable, lightly polish and remeasure. If polishes out and sizes within limits can be reused.
13) Measure alignbore and thrust cut for size.
14) From measurements, make sure you can obtain correct crank and rod bearings before doing any machine work. Once you are sure you can obtain proper parts then machine shop does the crank grinding if needed, align bore etc.
15) Fit parts to be sure everything still matches specs.
16) Do other things that wear like check lifter bores, cam, heads, surface where the cylinders mount, studs for missing or stripped, T4 syndrome, case plugs, pressure relief area, etc
17) balance all rotating masses
18) final machining on rods if much is needed to balance them
19) When you finally get around to reassembly use plasti-gauge to be sure that the bearing clearances are correct. You can do the rods and split shell bearings only with plasti-gauge.

bearings and pistons in the USA are generally measured by .010" oversizes. Example .010", .020", .030". Metric are measured in fractions of a mm. To make it easier on everyone world wide, metric bearings are made in .25mm oversizes because .25 mm = .00984" which is about 1/6 of 1/1000 of an inch different - or essentially .010" same as what we use in the USA.

Wed Nov 26, 2014 12:04 pm

It is important to note that you can not tighten up a sloppy thrust bearing by changing the shim thickness. The trust bearing should fit tight in the case and there must be a .004" +/- thrust clearance in the shim pack even if the thrust bearing is improperly not tight in the case.

Wed Nov 26, 2014 12:18 pm

SGKent wrote:
bearings and pistons in the USA are generally measured by .010" oversizes. Example .010", .020", .030". Metric are measured in fractions of a mm. To make it easier on everyone world wide, metric bearings are made in .25mm oversizes because .25 mm = .00984" which is about 1/6 of 1/1000 of an inch different - or essentially .010" same as what we use in the USA.

I think the rest of the world looks at it this way:

Bearings and pistons are generally measured by .25mm oversize and under size. Example .25, .50, and 1.00mm. Oddly enough the Americans use a different system and must converter the simple metric numbers to an "English" approximation which ironically is based on the number 10 unlike most any other measurement the Americans are used to. The simple .25mm metric under and over sizes (.25mm = .00984") are thus approximated for American use by noting them as .010".

:wink:

Wed Nov 26, 2014 12:35 pm

SGKent wrote: 19) When you finally get around to reassembly use plasti-gauge to be sure that the bearing clearances are correct. You can do the rods and split shell bearings only with plasti-gauge.

Then you could go a step farther and torque the clean case with the main bearings (no crank) installed. Then use a micrometer to measure main bearing diameters (vertical and horizontal) then mic the OD of the crank to get your main bearing clearence.

(I'd be lying if I said I didn't learn that this morning from Colin :o )

Thu Nov 27, 2014 9:02 am

Thank you all for your feedbacks! It was quite a value for me since I have zero experience in this. Now, there's another small thing I want to ask. It's about cylinder copper head gaskets, this thing:

Turns out, my engine is missing one pair, on left cylinders so I have to order new set of 4. The matter is, there are two dimensions given, for example 94mm diameter and 0.5mm thickness. Is the diameter outer one, or inner one? Which should I measure?[/img]

Thu Nov 27, 2014 9:19 am

Measure your pistons

Thu Nov 27, 2014 9:28 am

Wildthings wrote: Measure your pistons

Pistons? But they are 85.5 mm and there is no such gasket.

Thu Nov 27, 2014 9:49 am

Try these guys.
http://www.geersengineering.com/products#
Happy Thanksgiving
Tcash

Thu Nov 27, 2014 9:52 am

Oh I wish I had that much money!..

Anyway, back to my issue, does anyone know if the indicated diameter is outer, or inner, for cylinder head gasket? Pistons are 85.5 mm, there's no such diameter for gasket, my existing pair (for right cylinders) are 94 mm outer diameter. Is that what I want?

Thu Nov 27, 2014 10:06 am

Harbinger wrote: Oh I wish I had that much money!..

Anyway, back to my issue, does anyone know if the indicated diameter is outer, or inner, for cylinder head gasket? Pistons are 85.5 mm, there's no such diameter for gasket, my existing pair (for right cylinders) are 94 mm outer diameter. Is that what I want?

Just contact your vendor by phone and tell them what you need. Ordering over the internet is often not the best way. Also make sure both of your heads are the same, maybe gaskets were used on one side and not the other to make up for a step being on one head and not the other. Could also be a head volume issue. The Type 4 engine I am putting together needed different base shims on one side and not the other due to the block having been machined on one side only. It takes a lot of checking to get one of these old engines together right.

Thu Nov 27, 2014 10:11 am

Wildthings wrote: Harbinger wrote: Oh I wish I had that much money!..

Anyway, back to my issue, does anyone know if the indicated diameter is outer, or inner, for cylinder head gasket? Pistons are 85.5 mm, there's no such diameter for gasket, my existing pair (for right cylinders) are 94 mm outer diameter. Is that what I want?

Just contact your vendor by phone and tell them what you need. Ordering over the internet is often not the best way. Also make sure both of your heads are the same, maybe gaskets were used on one side and not the other to make up for a step being on one head and not the other. Could also be a head volume issue. The Type 4 engine I am putting together needed different base shims on one side and not the other due to the block having been machined on one side only. It takes a lot of checking to get one of these old engines together right.

I really wish I had a vendor in my country.