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305 specs

 
jonnyboy.z28
I love my Chevy Chevette!
Posts: 3
Joined: 03/05
Posted: 07/26/06 09:13 AM

I am rebuilding a 305 for an 85 Camaro Z28 (tpi) and would like to confirm cylinder bore and crankshaft journal dimensions for my machine shop as I have read conflicting information from various sources.


Bore: 3.736" Haynes manual lists tolerance between 3.7350 to 3.7385 but Chilton list 3.736" only for the low compression 305 and also list 3.740" bore for the high compression L69/LB9.


Main Journals: #1   2.4484"to2.4493"   #2,3,4   2.4481"to2.4490"   #5   2.4479"to2.4488


Rod Journals:   2.0986"to2.0998"


Also, the car is an 85 Camaro 305 tpi but I suspect the engine is a replacement and most likely a low compression 305, hence why it is important to verify the different bores for the two quoted by Chilton.


Thanks,


Jonathan    

 

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 07/27/06 10:30 AM

I have to wonder about your machine shop if this is the kind of question they're asking, this is SOP information for people who do this work for a living.


The difference between the Haynes manual and Chilton's is that Haynes is calling out the production limits which is the result of manufacturing tolerance which is some few thousanths of an inch about a nominal point. What Chilton is showing is a point that's 7/10,000 inch off the mid point between the high and low production tolerance. In fact factory liturature doesn't even deal with numbers this small when referencing the bore as 3.74 inch. The actual bore the shop performs should be based upon the piston diameter plus the desired clearance. So if you take a piston with a skirt diameter of 3.74 inch and add .0025 for clearance the machine shop should finish the bore diameter at 3.7425 inch. This is only an explanation of how the process and math works, I'm not giving you an explicit size they have to meet.


Bore diameter numbers of a few thousanths of an inch have no practical effect on compression ratio, they're just too small. Compression ratio were talking about here is called mechanical or measured or static compression, it really dosen't reflect what's going on inside the running engine where what's called the dynamic compression is variable to throttle opening, operating temperature, humidity, atmospheric pressure, RPMs, and cam timing to name but a few of the conditions. The measured compression ratio is the sums of the combustion chamber volume + head gasket volume + clearance volume of the distance between the piston crown and the head deck; and the volume if any of a piston dish which is an add, or a piston dome which is a subtract from these volume sums. One can go crazy and add things like the volume between the piston crown diameter and the bore diameter above the top ring if one wishes. The previous summed volume is then divided by the swept volume of the cylinder, which is simply bore and stroke dimension through the volume formula for cylinders. You don't need a bunch of fancy and expensive equipment like a digital Burette to make these measurements, a simple 100 mili-liter graduated cylinder will suffice, mili-liters and cubic centimeters are the same thing so the math is straight forward. For inch to cc measures there's 25.4 milli-meters to the inch and 16.4 cubic centimeters (16.387 plus some numbers if you want to get picky) to the cubic inch. This will get you within a couple/ 3 ccs and by the time all these numbers get integrated, that's plenty good enough.   


The crank journal sizes are much the same explanation, these are desired engineering/manufacturing dimensions. The variance by journal location reflects  an adjustment the various load conditions at those points. I'd be highly surprised if anything like that was a goal of the guy who set up the crank grinder. More likely than not, all the journals were originally ground to the same diameter regardless of location. Any exception to that will most likely be a number between .010 and .040 undersize indicating either a factory reworked manufacturing error, or an in-service repair that was made at some point during the vehicle's life.


Bogie 

 

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jonnyboy.z28
I love my Chevy Chevette!
Posts: 3
Joined: 03/05
Posted: 07/28/06 03:32 AM

Thank you for your explanation oldBogie.


A couple of points, firstly re machine shop: I have no doubts about the quality of their work as they have done work for me on several domestic engines, but here in England rebuilding a Chevy engine is a relative rarity for them. We have a handful of specialists who build race engines who are more experienced with Chevies, but what Im doing is only a basic service rebuild.


Secondly, re bore sizes: what I was trying to clear up is Chilton listing 2 different bore sizes as they state the 8.6:1 engine (LG4) is 3.736" but the 9.5:1 (L69/LB9) is 3.740"


Thanks, Jonathan

 

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 08/23/06 05:48 PM

Ahh the little details, I didn't realize you're in England, so I've got a better idea of your situation at the machine shop.


If that was an American shop I would have figured they thought they had a kid and were jerking him around.


Bogie

 

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