M20 Stroker Build: Take 2

[Lugnuts]

Going back and forth over a bearing clearance on the #6 main journal.

Choice of 0.047mm / 0.0018", or 0.067mm / 0.0026".

Should I go tighter or looser?

[ForcedFirebird]

.0018" is the high side of the factory tolerance - which is a little tight IMHO. Rule of thumb when building engines is .0001" for every 1" of journal diameter. .0026" is a little loose, even by "rule of thumb" standard (mains are 2.3"). This is the very reason I feel the s54 had rod bearing issues. They left the bearing clearance around .001" but increased the crank pin by a fair bit (and were already "tight" on the 1.7" pins).

I'd go with the .047mm/.0018"

[Lugnuts]

Thanks Firebird.

After measuring and remeasuring (and re-remeasuring!) the #6 clearance, the 2 sets of Kolbenschmidt mains are coming up too loose (0.075 / 0.0029"), and the BMW greens are just too tight for my liking (0.045 / 0.0017").

Based on the crank journal, the sweet spot is 60.035 / 2.3635...which the other mains come in at (or acceptably close to).

The set of ACL Race bearings I was sold push the clearance even further out of spec...0.085 / 0.0033".

Measured the main tunnels w/o bearings, caps torqued to 44 ft/lb - all pretty consistent (~65.015). Bearing shell thicknesses ranges from 2.503 (BMW green) to 2.493 (KS set 1) to 2.480 (ACL)...

Mixing bearing shell halves (green/yellow) is a no-go, since the other 6 mains are right on the money.

So, I'm thinking I might have a bad set KS mains (both sets were bought together a while ago). Put an order in for a pair of BMW std/yellow late model thrust bearings (cheaper than a full set from either Glyco, KS, or King)...fingers crossed!

[digger]

get some plastigauge (ACL flexi gauge is available locally) to double check what the mic's are telling you

[ForcedFirebird]

get some plastigauge (ACL flexi gauge is available locally) to double check what the mic's are telling you

Good catch. Haven't realized the bearing haven't been bolted down yet. Bearings change shape once clamped.

[digger]

Good catch. Haven't realized the bearing haven't been bolted down yet. Bearings change shape once clamped.

i meant as a 2nd opinion can get a slight difference between the method that might be favorable such that the tighter option because more palatable if it isnt already. he had the caps torqued down.

i'm pretty sure BMW recommendation for other engines such as S50 and the like are such that this would be in spec, so unless this a turbo engine or high rpm its probably ok at 0.0017"

[Lugnuts]

Got 3 strips of FlexiGauge. Confirmed my measurements yesterday - yellow is loose, greens is tight.

I also have a 2nd bore gauge for smaller bores, but the stem and dial are compatible - same numbers.

Even took the block of the engine stand so I can get at #6 from the back side - no difference.

Does the S50 run at a higher oil pressure? making a tighter clearance possible vs an M20?

[digger]

M20 spec is 0.0012-0.0028
S50 spec is 0.0008-0.0018

honestly itll be fine its a fairly mild build, id more worried about not being too far tight on the big ends

i think its the other way, the tighter clearance creates more oil pressure

[ForcedFirebird]

According to my literature (blue text means it was converted from metric, and note the typo for m20 green bearings)...



Tighter clearance at the bearings means more oil pressure somewhat on m20's, but I have found modifying the relief and pump is the way to increase pressure. I even had one crank on the tight side of the factory tolerance and pressure didn't make a noticeable difference (~16psi hot, ~65psi max cold [factory relief]). By adding spring shims to the relief and bypass in the pump itself the race car sees ~90psi cold and ~25-ish psi hot.

Don't be afraid to mix-and-match bearing half shells, either. It's actually common practice to use a shell from each color to get where you need it. In fact, when I was a kid and the 383 Chevy stroker was becoming popular, many builders would use 2 sets of bearings stacked on top of each other to pad the difference when mixing Ford rods, 400 crank and 350 block (or turn some spacers to half and pad) - took a few years for real kits to come out, but a very popular budget hot rod engine that was quite reliable.

We’ve also found that measuring all eight rod bearings will create a range of clearances that might span 0.0005-inch. Swapping bearings between the loosest and tightest can sometimes narrow the range of clearances. This idea can also be used on main bearings. Of course, substituting half bearing shells is another common way to bring the clearances closer to the desired spec. For example, the builder can compensate for a slightly oversize standard crank journal with a 1X bearing that offers more clearance. By adding one shell rather than both, this will increase the clearance by 0.0005 inch.

http://www.superchevy.com/how-to/engines-drivetrain/1904-engine-blueprinting-tips-part-1

[tiger_e30]

Plus 1 to forced firebird. This saved me a world of hurt when I was measuring main clearances and was coming back high per Bentley

[tiger_e30]

According to my literature (blue text means it was converted from metric, and note the typo for m20 green bearings)...

Tighter clearance at the bearings means more oil pressure somewhat on m20's, but I have found modifying the relief and pump is the way to increase pressure. I even had one crank on the tight side of the factory tolerance and pressure didn't make a noticeable difference (~16psi hot, ~65psi max cold [factory relief]). By adding spring shims to the relief and bypass in the pump itself the race car sees ~90psi cold and ~25-ish psi hot.

How do you actually add the spring shims to the relief and bypass? Are you basically just augmenting the force required for them to open?

[digger]

why would you even do that......... if there is nothing wrong with the bearing clearances or anything else mechanically that affects the oil flow path / leakage paths whatever the resulting oil pressure ends up it ends up, all you need to do is ensure the operating oil temp is appropriate for the oil viscosity.

the big thing that kills bearings is mostly interupted oil supply to the pickup and subsequent lack of oil flow rate to the bearings, aerated oil and things of that nature, its not lack of oil pressure in the sense of normal operation of the pump and pickup.

[Lugnuts]

Don't be afraid to mix-and-match bearing half shells, either.

I actually tried this a week or so ago. With a green + yellow I get a good clearance.

Only issue is, as I understand, if you mix and match bearing shells for one main, you're supposed to do it for all mains - the thicker shell on the bottom half will raise the crank centre line.

I don't have enough experience to know if that tiny difference is something I can safely get away with?

Putting greens across the other 6 mains will tighten clearances by 0.01mm/0.0003"...which should be irrelevant for my DD use case...

[digger]

https://www.speed-talk.com/forum/viewtopic.php?t=22401

[ForcedFirebird]

As the link says Digger posted, you can add the shells wherever. Been a long time ST member, good stuff over there. I like what "Baprace" and "Wolfplace" had to say about it:

"I have used different size bearing shells from the same manufacturer and when I use a std on top and a .001 on the bottom there are no problems and on next rebuild I use the other 1/2 sets and no problem there either. Now if i am using one journal different I always try and put the smaller shell on the top so the "line bore" remains the same.
I have used different size bearing shells from the same manufacturer and when I use a std on top and a .001 on the bottom there are no problems and on next rebuild I use the other 1/2 sets and no problem there either. Now if i am using one journal different I always try and put the smaller shell on the top so the "line bore" remains the same."

Same here
My opinion it does not make a lick of difference which half you change.
The damn crank probably looks like a piece of spaghetti when running anyway

The only thing I do not do is put one in up & one down in the same engine
If I change one or more I always change the same, halves be it top or bottom

Consider you are moving the centerline by a whole 2 1/2 tenths,,,,
I seriously doubt the engine will know the difference

How do you actually add the spring shims to the relief and bypass? Are you basically just augmenting the force required for them to open?

Correct.

why would you even do that......... if there is nothing wrong with the bearing clearances or anything else mechanically that affects the oil flow path / leakage paths whatever the resulting oil pressure ends up it ends up, all you need to do is ensure the operating oil temp is appropriate for the oil viscosity.

the big thing that kills bearings is mostly interupted oil supply to the pickup and subsequent lack of oil flow rate to the bearings, aerated oil and things of that nature, its not lack of oil pressure in the sense of normal operation of the pump and pickup.

We only do it on the endurance cars since the oil supply does get interrupted at times, aerated, as well as loss of viscosity. Even with the increased pressure (and volume) the 10w60 oil is like runny water at room temp by the time we have been at or near WOT for 12-24hr straight. No need on a street car - as you said, those operating temps/abuse is not what it is in an endurance car. I was more aiming to the fact that .0001" isn't going to change the pressure significantly.