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Old 02-01-2009, 02:01 AM
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72' sbc 350 going in a 92 GMC sierra

I'm new here and am kinda just looking for some feed back on my rebuild. I have a 72' 4-bold main block. When I finally took it out of the truck it was only running on about 4 or 5 cylinders and smoking really bad. I am looking for a 9.2-9.6:1 CR and 350-400HP moderate mid range torque would be nice as well. This engine is going in my 92 GMC sierra ex-cab short box 2wd. For the engine I am going to have it bored .030" over and maybe .015" taken off the decks. I have decided honing isn't going to cut it with this rebuild. I will run a stock crank and stock connecting rods(they will be balanced). I have the World products sportsman II heads 1.25" springs 100# seat and max lift @ .560". I am having a hard time finding the right cam. I know the cam is very important and I want everything to run smooth the first time. I have learned a lot by reading through a lot of posts but nothing really for what I have. I will continue to find out as much as I can through my own research but what I am having a hard time understanding is the cam/CR/pistons. pick the cam figure your compression ratio and then pick your pistons for the compression ratio. I guess I need to go study the formula for the CR more. I know what the numbers for the different cams mean but am having hard time grasping how they each effect HPs and torque differently. The next engine I build will be of better quality and I am hoping to know exactly what I going to put into it and why I am putting it in and what it's going to do. Any site that could help me would be most appreciated also. I have been having trouble finding good information websites on this kind of stuff. Thanks for you patience with these questions. All feed back is very much appreciated.
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Old 02-02-2009, 07:40 PM
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Quote:
Originally Posted by njones308
I'm new here and am kinda just looking for some feed back on my rebuild. I have a 72' 4-bold main block. When I finally took it out of the truck it was only running on about 4 or 5 cylinders and smoking really bad. I am looking for a 9.2-9.6:1 CR and 350-400HP moderate mid range torque would be nice as well. This engine is going in my 92 GMC sierra ex-cab short box 2wd. For the engine I am going to have it bored .030" over and maybe .015" taken off the decks. I have decided honing isn't going to cut it with this rebuild. I will run a stock crank and stock connecting rods(they will be balanced). I have the World products sportsman II heads 1.25" springs 100# seat and max lift @ .560". I am having a hard time finding the right cam. I know the cam is very important and I want everything to run smooth the first time. I have learned a lot by reading through a lot of posts but nothing really for what I have. I will continue to find out as much as I can through my own research but what I am having a hard time understanding is the cam/CR/pistons. pick the cam figure your compression ratio and then pick your pistons for the compression ratio. I guess I need to go study the formula for the CR more. I know what the numbers for the different cams mean but am having hard time grasping how they each effect HPs and torque differently. The next engine I build will be of better quality and I am hoping to know exactly what I going to put into it and why I am putting it in and what it's going to do. Any site that could help me would be most appreciated also. I have been having trouble finding good information websites on this kind of stuff. Thanks for you patience with these questions. All feed back is very much appreciated.
Honing and lapping, with the exception of fine finish work today, belongs to an automotive era starting 60 years ago and going backward into geologic history from there. I was used on flathead engines running cast in place Babbitt bearings for vehicles that never went faster than 40 MPH and probably never saw the high side of 20,000 miles without a major rebuild.

Your truck came with a Throttle Body Injected (TBI) engine. It and the 72 don't have too much in common. If you're keeping the injection, it will not like changes in compression, most definitely cam timing and lift, things like that will upset it, not to mention it's intake won't bolt to 72 heads without doing some carving on the manifold. This of course can be gotten around with a carburetor, then you'll have to change the fuel system as the EFI in tank pump delivers too much pressure for a carb. But this is manageable with a regulator or discontinuing the electric pump and using a mechanical on the 72 block to move gas. Your truck's flexplate or flywheel will not bolt to the 72 engine, you'll have to use the older flywheel that is compatible with the business end of a 2 piece rear seal crank. Fortunately GM didn't change torque converter or clutch patterns so this goes together real nice with the right parts.

I've seen another blog of yours which causes me to explain cylinder boring.

Honing is a finish operation used to establish the cylinder wall clearance to the piston and to apply the correct wall finish for the type ring being used, chrome a course finish, unplated or coated rings take a medium finish especially with a cast oil ring rather than stainless, and a fine finish for a moly ring even with a stainless oil ring in the package. honeing is done to a cylinder wall that has been dimensioned and positioned by a maching using a boring bar, of which tere are several types.

Boring is a ridged machining operation that not only needs to make a uniformly round cylinder wall, but that wall needs to be perpendicular to the center line of the crankshaft. As engines wear not only does the bore become out of round and tapered but the centerline also moves someplace besides aimed at the center of the crank. Some of this is because wear is rather random in where it occurs but some of this is also due to residual casting stresses being relieved or relaxed over time, this is especially prevalent in engines that have been overheated.

There is cheap boring and expensive boring. Cheap boring uses a tool that aligns using the head bolt pattern and deck, it gets the bore round and removes the taper but does nothing to aim the bore at the crank's centerline. This process assumes the head deck and the main bearing saddles are correctly positioned relative to each other. But when you go back to casting and operational stresses working out of the casting over it's life and the possible dimensional changes from overheating, this becomes an assumption firmly rooted in nothing firmer than standing in sand.

The expensive, but precise way, starts with aligning the main bearing bores, then paralleling the head deck to that reference, then boring the cylinders with an alignment tool that fastens to, or optically aims, the boring bar to a mandrel placed in the main bearing bores. The cylinder bores are then aligned to where the rconnecting od centers on its journal for each cylinder and at the center of the crank's mains. This fixes front to rear alignment of the bore, and makes it 90 degrees to the crank center, so the bore ends up properly located in 3 dimensional space to the crank and the head deck. The boring should also be done with a torque plate that simulates the stresses of the cylinder head. the reason is the changes in how engines are built today compared to 60 years ago. In the good old days of really thick castings the head bolt bosses were put in the exterior sides of the water jackets. This puts the head bolts a considerable distance from the cylinder wall so it wasn't distorted by the bolts clamping the head down. The head gasket could keep a seal because there was a half inch of cast iron in the deck with another in the head making a very stiff surface. With the reduction in casting thicknesses that started in the 1950s, these surfaces were no longer stiff enough to hold the head gasket so new designs moved the head bolts to the cylinder walls to reduce the distance between the bolts. The problem this causes is that the cylinder walls are pulled out of round by the clamping forces transmitted by the head bolts. The torque plate is used to simulate these loads while the cylinders are being bored. So the finished cylinders with the heads on will be truer in their roundness and taper than otherwise possible.

While this adds cost it also adds may miles to a rebuilt engine. Without these techniques, a rebuilt engine seldom comes close to lasting as long as the original factory built engine does.

Bogie
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Old 02-06-2009, 05:30 AM
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Thanks very much for that info bogie. It helps a lot. The description of the honing precces's helped greatly. My resources up here in alaska are not that great. But I get by. I decided to hone my engine with a drill hone for now, and just put it together with what I have see how it runs. And then start ordering correct parts for my next rebuild. Trial and error is the best way to learn I think. You can read all sorts of stuff in books but it only goes so far. Hands on is the best. I hope to have this first engine done in a week or two. If eveything goes as planned. Thanks again bogie.
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