quote:
Originally posted by jsharp:
quote:
Originally posted by KW:
I'll have to dig out some of my newer books but I don't buy in to the clearance thing at all.
1971 350 Chevy crank and rod.
.0008-.0020 and .0013-..0035
1985 350 Chevy crank and rod.
.0008-.0020 and .0013-.0035
0.
I don't buy it either. Unless they've gone to more exotic materials or changed the sizes of the parts drasticly the clearances shouldn't be that different...
Your instincts serve you well on main bearings and not so badly on rod big ends.
Here's a couple more data points from factory shop manuals. If I had given clearances as (inch clearance/inch bearing diameter), the results might have been even closer. But the manuals are put away now.
2000 Acura 3.2L V6 crank and rod (inches)
0.0008 - 0.0017 (1) and 0.0008 - 0.0017
1996 Chevy 5.7L V8 LT-4 crank and rod (inches)
0.0009 - 0.0024 and 0.0013 - 0.0035
Note (1) Bearings 2,3,4
On the mains, all three Chevys and the Acura are all pretty close at tightest clearance. The Acura isn't as loose on the loose end. That's because Acura holds their manufactuing tolerances closer. If you asked the 1971 GM engineers what they really would have liked to use for main bearing clearances on the loose end of the tolerance range, I'll bet they would give you numbers closer to Acura's than GM's.
As far as the range of main bearing clearances goes, the 1971 Chevy oil has to handle clearances just as small at the newer Acura's oil.
Con rod big end clearances are another story. All three Chevys are the same at 0.0013 to 0.0035, a range of 0.0022 from tight to loose.
The total tight to loose range on the Acura is 0.0009. Only 41% of the Chevy's tolerance range. Looks like Acura sees a reason to hold those clearances a lot closer than Chevy and is willing to pay for the extra manufacturing control.
We could ponder why Chevy held their crank tolerances in 1971 almost as well as Acura did in 2000, yet let their rid big end tolerances go all the h3ll compared to a more modern engine.
One other little factoid to toss into the fray that not everyone is aware of....
Within reasonable real world limits, the bigger the static bearing clearance, the smaller the running bearing clearance on the loaded side of the bearing.