Loose tolerance engine

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It's not over my head, it just seems to be an illogical way to present the numbers. Every drawing for our parts has a dimension for the part and at the bottom a specific tolerance depending on the number of decimal places in the dimension. If it deviates from that, the tolerance will be stated in the dimension. I'm sure most every industry drawing is this way.
For example we have a 12.75" OD endcap, we put 12.75 and use the tolerance on the bottom (+/-.030). We do NOT call the endcap a 12.72-12.78" endcap. That is silly. What is the target?
 
I would imagine that you would want very tight bearing clearances in a racing engine. The crank rides on a thin film of oil and never touches the bearings. Increased clearances would likely put allow movement and put pressure on the oil film, either shearing the film, or pushing the film into the bearings (imagine tapping on a bearing with a ball peen hammer causeing the ends to curve inward towards the crank). If you detonate, something will break, period. A fully forged professionally built reciprocating assembly will only buy you a few seconds at best. I don't see how increased bearing clearances will help you in that situation. A steady adequate flow of oil is all you need. Anything beyond that decreases horsepower, increases stress on distributor, cam and timing chain (or gears, belt) and increases chances of pumping the sump dry. Grooved bearings decrease bearing surface area and I would imagine give the oil an escape route away from the bearing surface.....things you don't want happening in an engine.

What does this have to do with what the cutoff is for "tight" or "loose" clearances?

You never answered my question on your cam selection. I know Bob chased us off the thread, But you should have PM'ed me. I was wanting to get the lowdown from a valvetrain specialist.
 
quote:
For example we have a 12.75" OD endcap, we put 12.75 and use the tolerance on the bottom (+/-.030). We do NOT call the endcap a 12.72-12.78" endcap. That is silly. What is the target?
Click to expand...
The data I found for my engine came from a repair manual. I presume it's easier for most people to compare what they measure with a given range than calculating.
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I still don't know when an engine would be called "loose."

MolaKule was the one who was compiling a list of engine clearances data for various makes, but unfortunately, he hasn't chimed in.


PS: Bob chased a few people off the thread? What's going on behind the curtains?
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[ August 28, 2003, 01:51 PM: Message edited by: moribundman ]
 
quote:
It's not over my head, it just seems to be an illogical way to present the numbers. Every drawing for our parts has a dimension for the part and at the bottom a specific tolerance depending on the number of decimal places in the dimension. If it deviates from that, the tolerance will be stated in the dimension. I'm sure most every industry drawing is this way
Click to expand...
if you have ever built an engine then this "clearance" way of measuring makes complete sense. you are not measuring the size of one part as in a drawing, you are measuring the relative distance (clearance) between two rotating parts reguardless of their actual size. usually with a special tool called "plasti-gage".
 
sbc350gearhead, thanks for clarifying this. I understand how a thread starts to drift from one topic to another. I think it's normal and can be difficult to avoid.

Anyway, can we get some hard figures on what's a motor with tight or loose clearances? Next we should compare oil requirements for each type of engine.
 
I have compared dozens of japanese and american engines in the last few hours, and all the specs for clearances are virtually identical (tolerances seem to favor the japanese engines by a small margin). I even looked up the specs on 30 year old SBC's and BBC's. The Thirty year old specs are virtually identical to the new ones (BBC's are a hair looser.....but not by much). Perhaps to have loose clearances, the engine must be worn out.
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Now I wonder if we are on a wild goose chase. Maybe the clearances on all engines are pretty close, and it is something else that determines what the best viscosity is for a particular engine.

[ August 28, 2003, 06:49 PM: Message edited by: sbc350gearhead ]
 
SBC;
The cam thing ended up in the motorcycle STP? thread.
Hint on all of the above, this thread:
You have .001" clearance in your rods 'n mains all minimum, but with in specs.
Your crank flexes a bit, the bolts stretch a little, the block heats up.
What is your clearance now?
Hey!!! What are you doin' readin 'my profile?
nono.gif
 
Jason;
Try this.....you have a journal inside a housing, just like inside SBC350's engine.
The crank journal on this types of engines are around three inches.
Production machining can keep a 3" journal to about .00025.
You friend works in a ship's engine room.
The crank journal is 30" in diameter.
The best machinist can only keep the run-out on this crank to .0025"
The rest of the story and answers are in the links of the links Molakule posted.
 
quote:
Originally posted by userfriendly:
SBC;
The cam thing ended up in the motorcycle STP? thread.
Hint on all of the above, this thread:
You have .001" clearance in your rods 'n mains all minimum, but with in specs.
Your crank flexes a bit, the bolts stretch a little, the block heats up.
What is your clearance now?
Hey!!! What are you doin' readin 'my profile?
nono.gif

Click to expand...
Well since the bearings will expand more than the other metals, and the crank journals will expand as well, and the cast iron and or steel used in the block and caps, will expand more than the 210k main studs used in racing apps.....I would imagine that the clearances would actually become smaller.
 
You missed the flex part.
Here is another one:
You have your crank done at 10/10 min.
Good job at .00025" +-
The rods are also min .00025" +-
The biggest rod happens to go on the smallest journal, and instead of .001" you end up with
.0015". No big deal.
The smallest rod just happens to go on the largest journal, and instead of .001" You have??
 
Mozart and Ludwig (Ludevig) knew about dominant and tonic notes.
When acting together these notes create_____________.
Add that to the crank flex and rod and main bolt stretch and subtract the possible total effect from .0005"
Then take a look at the post I did not quite finish in the question of the day section.
The topic was
Film thickness or something like that.
 
My mother always told me NOT to go out with a loose tolerance engine and I have tried very hard to honor her wishes.
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SBCGear.....
Ok we're down to .0005 on the rods and not taking into account flex or harmonics.
The stock block is not bad in the main saddle area, with only +- .0005 from the highest to the lowest.
What is your clearance on the tighest main if you were shooting for the minimum .001"?
The object of engine "blue printing" is to take out all of those little fractions that can ruin a high performance engine in the first 10 minutes.
Opening up the clearances to maximum is a band-aid approach, but a necessary one.
Four bolt splayed blocks, forged cranks, high grade bolts and studs.
All of those things are used to keep flex down.
Detonation is likened to hitting the top of the piston with a hammer.
Some where on this board someone is blaming the brand and grade of engine oil for his ruined rod bearings.
What do you think of that?
 
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