When it comes to engine work, it is essential to measure to a tenth of a thousandth. A good engine is all about super precise measuring and fitting, as you’ve found out.
You’ll have a good engine when this is all finished.
Metric Mechanic Roadtrip
FishMan
#22
I’m sitting on the couch right now listening to Jim from MM and Scott. They are talking about oil; this is getting very interesting.
Ranger
#23
To put the precision in perspective for the art history majors, a hair’s thickness is ~3 thousandth. So a 10kth is a 30th of a human hair. And if you’ve got 4 lesser graduations than you’re working in 120th of the thickness of hair.
PDS
#24
great, now share the secrets…
Main bearing clearance?
Rod bearing clearance?
Piston to cylinder Clearance?
Thanks!!
turbo329is
#27
Ranger wrote:
about a year ago we decided in my shop “a cunt hair” needed a real value. 8 thousandths. No one was interested in the thickness of a head hair so we didn’t measure one.
Ranger
#29
PDS wrote:
[quote]great, now share the secrets…
Main bearing clearance?
Rod bearing clearance?
Piston to cylinder Clearance?
Thanks!![/quote]
There’s nothing secret really, and there’s nothing unusual about the clearances, it’s just an obsession with precision. Like rod bearings, according to Bentley, are supposed to be .0012 to .0028" IIRC. But Jim says that if you have a rod at .0028, it would suck much of the oil out of the engine and there’d be starvation problems in all rod bearings.
There is a generic bearing rule of thumb that says 1 10kth per inch of journal bearing. The rod journal is ~1.8" so in a perfect world the the bearing gap would be 18 10kth, or .0018. So instead of a sloppy bearing tolerances like the Bentley seems to encourage, MM keeps at it until they have every rod bearing to their desired bearing gap, not just to some range of tolerance. Maybe .0018" for a DD and .0020" for a race car.
So it’s not a matter of doing something tricky, it’s a matter of being obsessed with precision.
Re. Metric measurements. American machinists grew up with the USCS system, sometimes incorrectly called SAE. Therefore they tend to be most comfortable there. But MM’s favorite dial bore gauge is metric and has graduations down to 10 micrometer. So every 2 1/2 on that dial bore gauge is .0001".
Therefore we were routinely measuring fractions of a ten thousandth.
PDS
#30
Gotcha,
I’m familiar with those specs…However, what separates the high end builders, other than the extreme precision, is that they work with specific tolerances that through experience results in maximum hp vs longevity…
Maybe those specs are held close to the vest.
Thanks again…
FishMan
#31
PDS wrote:
[quote]
Maybe those specs are held close to the vest…[/quote]
Like Scott said, Jim isn’t showing us any secret tolerance or crazy tricks, we are just achieving very precise clearances and making sure every journal is exactly right. The thousandth per inch of bearing journal rule is what we use. Piston clearances are different than I imagined. Because the M20 engine runs hotter on the front and back than in the middle, piston clearances on each end are slightly greater than the center pistons.
Working in Jim’s shop is not something I can reproduce in my garage. He has a special tool for every step of assembly an and uses his vast array of tools and equipment to make sure every part fits perfect. If the surface finish of a pin isn’t exactly right, he puts the pin in lathe and smooths it out. We checked pistons for burs and made sure they were completely smooth. He has special spring compressors that he custom machined for our stock pistons and another special tool that smooths the wrist pin holes. The neatest thing I’ve seen, Jim showed me when I stopped by over Christmas. He has a tool that uses compressed air to place the combustion chambers under vacuum and indicates how well the valves seal.
Before we work on any part, everything must be perfectly clean. He has a custom built assembly that sprays hot water and cleaning agents on parts as the rotate on a pedestal. Before using any part, we clean with water or solvent, glass bead if necessary, clean again, and then dry with compressed air. Using the MM building process, there simply isn’t any chance for failure; everything must fit exactly perfect or you adjust and and change things until it does.
Jim mentioned a few failure points that some may be overlooking. When road bearings pound from excessive wear or oil starvation, it may send shrapnel through the entire engine. Most would probably expect to find chunks of bearings in the oil pump but may not consider that bearing bits could be lurking in your oil cooler just waiting to infiltrate your engine after a fresh rebuild. Oil bits could also find there way inside the cylinder head and severely score the cam bearing journals; if this were to happen, you could have low oil pressure under some conditions as oil would leak from the cam journals. My head ended up having this very problem and Scott took apart a few oil pumps that were full of metal chunks.
PDS
#32
Great info…I agree that us home builders will never achieve that level of precision, but that was great info and I have a new level to aspire to …
Thank You Thajnk You Thank You!!!
cwbaader
#33
Now I hope everyone will understand the time required to assemble a motor. As Ranger and FishMan said, there is major time required to clean, check, measure, and assemble. 100 hours would not be unusual. So, when you look at the price of a motor rebuild, consider about 3-5k of that to be labor. Chuck
Steve_D
#34
FishMan wrote:
9.3.1.2.1. Machining for balancing purposes only is allowed.
Wrist pin balance is critical, after all. 
Ranger
#35
I was using used pistons in order to save some bucks. Some of them had burrs on them and that caused some scoring in the cylinders. Didn’t notice it until the block was all assembled. Was really a bummer. Jim had to rehone the block. That was the only real goatscrew of the 5 day effort.
Last night we didn’t knock off until 1AM. I left MM this morning at 05 and got home 17hrs later.
Was really a helova experience. And I can’t say enough about the graciousness of Jim and Mary taking us into their house. It was over the top. How over the top was it? When Fish and I ran out of beer Mary ran out and got us some more. The only thing that was missing was the cruise ship.
dherrington
#36
How do you think the end product compares with a crate engine from BMW? Do you know what kind of tolerances they achieve in their manufacturing process?
FishMan
#37
dherrington wrote:
That’s a great question! I don’t think you would ever know without disassembling a crate engine. The advantage of what we did is that we know exact clearances throughout the motor and adjusted everything to specific targets. BMW may have a larger acceptable range…
Scott, I’m not convinced the piston skirts caused the scratches on your cylinder walls… I think the center rings caused the scratches. Jim and I discussed while we adjusted my valves yesterday morning.
Anyway, the whole experience was tremendous and like Scott said, Metric Mechanic was very kind to us! I also have a new appreciation for the detail involved in motor building. Personally, I have about 60 work hours in my engine. When Scott and I arrived, Jim had spent the previous week preparing for us. I’d estimate 100+ hours for a proper engine build…
Ranger
#38
dherrington wrote:
The short answer is I don’t know. All I know for sure are the specs in Bentley. But I think that we misinterpret the Bentley guide in this respect. For example, we look at the rod bearing tolerance spec of .0012 to .0028 IIRC and think to ourselves “When I build an engine, I need to be in that range and I’ll be good”.
When I first attempted to build a bottom end that is how I interpreted the specs.
But maybe what that spec is really for is to measure the health of an old engine. Maybe BMW really aims for a .0018 to .0020 and would burst out laughing if someone suggested that .0024 would be ok for a new motor.
So I can’t tell you what the specs are on a new crate motor. But I can tell you that MM has a lot of respect for BMW manufacturing, even if there are design elements that are non-optimum for racing. MM doesn’t give that respect easily.
jlevie
#39
The specs published for clearances are usually taken as wear limits. The manufacturer works to a more narrow band of clearances and with modern manufacturing and quality control tools that can easily be achieved. That is one of the big reasons that modern engines go way past 100k. It wasn’t that long ago that a 100k engine was one that was about to be rebuilt or suffer catastrophic failure. And I’m talking street engines, not race engine.