13 replies to this topic

[BaronVonchesto]

Hello miniforum friends,

 

I finally was able to get hold of a 12G82 crank to start my 1098 project (though the number on the crank is 12g82A, which i suspect means factory cross drilled, since this is a cross drilled crank).

 

Now taking the measurements, its pretty obvious that the crank will need oversied bearings. Im not very confident in my ability to use a micrometer accurately, but here are the measurements I got of the various journals:

 

Main Journals:

Closest to flywheel: 43.94mm

Middle: 43.94mm

timing pulley side: 43.95mm

 

Big Ends:

No 1: 40.71mm

No 2: 40.67mm

No 3: 40.70mm

No 4: 40.65mm

 

From the Haynes manual: for 85H,95H and 10H

Main bearing journal diameter: 44.46 - 44.47mm

Minimum main bearing journal regrind diameter: 43.45mm

 

Big end journal diameter: 41.28 - 41.29mm

Minimum big end journal regrind diameter: 40.27mm

 

 

The machine shop says I need +30 for the mains and +40 for the big ends. Is this true? how do I validate this?

[Spider]

Hello miniforum friends,

 

I finally was able to get hold of a 12G82 crank to start my 1098 project (though the number on the crank is 12g82A, which i suspect means factory cross drilled, since this is a cross drilled crank).

 

Now taking the measurements, its pretty obvious that the crank will need oversied bearings. Im not very confident in my ability to use a micrometer accurately, but here are the measurements I got of the various journals:

 

Main Journals:

Closest to flywheel: 43.94mm

Middle: 43.94mm

timing pulley side: 43.95mm

 

Big Ends:

No 1: 40.71mm

No 2: 40.67mm

No 3: 40.70mm

No 4: 40.65mm

 

From the Haynes manual: for 85H,95H and 10H

Main bearing journal diameter: 44.46 - 44.47mm

Minimum main bearing journal regrind diameter: 43.45mm

 

Big end journal diameter: 41.28 - 41.29mm

Minimum big end journal regrind diameter: 40.27mm

 

 

The machine shop says I need +30 for the mains and +40 for the big ends. Is this true? how do I validate this?

 

These engines are imperial, not metric. The Bearings, Pistons etc are nearly always over or undersized in blocks of 10 thousands of an inch.

To me, when measured in metric, is a source of frustration and another place for errors to creep in. Assuming the measurements are correct and my translations are error free, these are the numbers you have;-

 

Main Journals:

Closest to flywheel: 43.94mm    1.79299"

Middle: 43.94mm                       1.7299"

timing pulley side: 43.95mm      1.7303"

 

Big Ends:

No 1: 40.71mm                          1.6027"

No 2: 40.67mm                           1.6012"

No 3: 40.70mm                           1.6024"

No 4: 40.65mm                           1.6004"

 

To work out what size bearings you need first we need to know the standard size ranges;-

 

Mains          1.7505 to 1.7501"
Big Ends      1.6254 to 1.6259"

 

Then subtract the sizes you have from these figures and that's your undersize.

[stoneface]

Just in case it's not spotted.

Main Journals:

Closest to flywheel: 43.94mm    1.79299" should be 1.7299"

 

[Spider]

Case in point !
 

Just in case it's not spotted.

Main Journals:

Closest to flywheel: 43.94mm    1.79299" should be 1.7299"

 

[monala]

What the machine shop is doing is comparing your measured journal sizes to the standard factory spec and then matching the closest undersize bearing set, since oversize in crank terms usually means undersize bearings to compensate for a ground-down journal. From your numbers, your mains at ~43.94–43.95 mm are roughly 0.5 mm under the nominal 44.46–44.47 mm, which lines up closely with a +0.030" (or ~0.76 mm total undersize diameter range depending on system used), so +30 mains sounds plausible but should be verified against the actual bearing Manufacturer catalog spec rather than just subtraction. For the big ends, your readings around 40.65–40.71 mm compared to 41.28–41.29 mm suggest a similar wear/grind situation that often lands near +0.040" bearings, so that call also seems reasonable. The correct way to validate it is to mic each journal accurately in multiple directions, confirm out-of-round and taper, then match the final ground size to the bearing manufacturer’s undersize range rather than relying only on the raw difference, because the clearance is ultimately set by the installed crank + rod or main housing + bearing + oil clearance at https://www.bearingforworld.com/, not just the crank measurement alone.

Edited by monala, 05 June 2026 - 01:42 AM.

[BaronVonchesto]

 

Hello miniforum friends,

 

I finally was able to get hold of a 12G82 crank to start my 1098 project (though the number on the crank is 12g82A, which i suspect means factory cross drilled, since this is a cross drilled crank).

 

Now taking the measurements, its pretty obvious that the crank will need oversied bearings. Im not very confident in my ability to use a micrometer accurately, but here are the measurements I got of the various journals:

 

Main Journals:

Closest to flywheel: 43.94mm

Middle: 43.94mm

timing pulley side: 43.95mm

 

Big Ends:

No 1: 40.71mm

No 2: 40.67mm

No 3: 40.70mm

No 4: 40.65mm

 

From the Haynes manual: for 85H,95H and 10H

Main bearing journal diameter: 44.46 - 44.47mm

Minimum main bearing journal regrind diameter: 43.45mm

 

Big end journal diameter: 41.28 - 41.29mm

Minimum big end journal regrind diameter: 40.27mm

 

 

The machine shop says I need +30 for the mains and +40 for the big ends. Is this true? how do I validate this?

 

These engines are imperial, not metric. The Bearings, Pistons etc are nearly always over or undersized in blocks of 10 thousands of an inch.

To me, when measured in metric, is a source of frustration and another place for errors to creep in. Assuming the measurements are correct and my translations are error free, these are the numbers you have;-

 

Main Journals:

Closest to flywheel: 43.94mm    1.79299"

Middle: 43.94mm                       1.7299"

timing pulley side: 43.95mm      1.7303"

 

Big Ends:

No 1: 40.71mm                          1.6027"

No 2: 40.67mm                           1.6012"

No 3: 40.70mm                           1.6024"

No 4: 40.65mm                           1.6004"

 

To work out what size bearings you need first we need to know the standard size ranges;-

 

Mains          1.7505 to 1.7501"
Big Ends      1.6254 to 1.6259"

 

Then subtract the sizes you have from these figures and that's your undersize.

 

 

Thanks Spider.! I gave metric numbers because my measurement tools are metric, as are the numbers in the Haynes manual.

Going by the converted numbers and subtracting from the standard size gives me:

 

Mains: 19.8 - 20.6 thou

Big ends: 22.7 - 23.2 thou (no 1) or 25.0 - 25.5 thou (no 4)

 

so I guess i should be getting +20 oversize bearings for both main and big ends?

[stoneface]

I would say you need a regrind to +30 

The dimensions you have measured vary quite a lot that a regrind would be advised.

[Ethel]

What the machine shop is doing is comparing your measured journal sizes to the standard factory spec and then matching the closest undersize bearing set, since oversize in crank terms usually means undersize bearings to compensate for a ground-down journal. From your numbers, your mains at ~43.94–43.95 mm are roughly 0.5 mm under the nominal 44.46–44.47 mm, which lines up closely with a +0.030" (or ~0.76 mm total undersize diameter range depending on system used), so +30 mains sounds plausible but should be verified against the actual bearing catalog spec rather than just subtraction. For the big ends, your readings around 40.65–40.71 mm compared to 41.28–41.29 mm suggest a similar wear/grind situation that often lands near +0.040" bearings, so that call also seems reasonable. The correct way to validate it is to mic each journal accurately in multiple directions, confirm out-of-round and taper, then match the final ground size to the bearing manufacturer’s undersize range rather than relying only on the raw difference, because the clearance is ultimately set by the installed crank + rod or main housing + bearing + oil clearance, not just the crank measurement alone.

 

Why I trust this stuff to the machine shop: and expect them to be responsible for doing the whole job properly.

 

Even if I could manage that, getting the journals in the best relationship to each other is more than difficult without a mill bed, or lathe, as a reference. 

[Spider]

Thanks Spider.! I gave metric numbers because my measurement tools are metric, as are the numbers in the Haynes manual.

Going by the converted numbers and subtracting from the standard size gives me:

 

Mains: 19.8 - 20.6 thou

Big ends: 22.7 - 23.2 thou (no 1) or 25.0 - 25.5 thou (no 4)

 

so I guess i should be getting +20 oversize bearings for both main and big ends?

 

 

As Stony has suggested - and not having seen it - I'd have it reground on the Mains and Big Ends to the next size.

Size and shape are one aspect here, another is surface finish and the size / shape of the radii.
 

[BaronVonchesto]

thanks all. The reason i was trying to measure the bearings is precisely because the machine shop suggested to get the bearings first then he'd regrind the crank. So I guess I'll be ordering +30 main and big ends then.

[gazza82]

thanks all. The reason i was trying to measure the bearings is precisely because the machine shop suggested to get the bearings first then he'd regrind the crank. So I guess I'll be ordering +30 main and big ends then.

Seems an odd way of doing things. Normally they would tell YOU what size they need to grind to first ...

[timmy850]

But I can also understand that they want to see the bearings installed, check the actual bearing measurements, then grind the crank to get the right oil clearance. 

 

I can also imagine it's a bit harder to get A-Series parts and machining done in Kuala Lumpur

[Ethel]

Certainly there's no point in grinding the journals to fit unobtainable shells, but I'd like to think I was buying the machinist's expertise to produce a good engine. If I was sourcing the parts myself I'd hope they'd do an evaluation & offer some advice on what  to buy.

[BaronVonchesto]

we don't have machinists specialising in mini engines here, but there are a few who are pretty experienced in handling mini engines. My regular guy no longer works at the machine shop I usually use. This time its a different guy who took a look at the crank. He recommended +30 mains and +40 big ends, but said its best to get the bearings first so he can measure everything together before he grinds the cank.