2.8 punched .75 over??? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 09-13-2000, 11:21 PM
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2.8 punched .75 over???

Well After putting all this time into what was supposed to be a crate motor with under 20,000 miles on it I am have more and more problems showing old age. After completely redoing the top end it is looking more and more like my rings are going out. So I started thinking about rebuilding a motor I had sitting around. Tonight I pulled the heads off and the oil pan off, First thing I noticed was the pistons were stamped .75 and they looked quite a bit bigger then stock pistons. is it possible that they were punched out .75 over? The only problem with this motor is the crank has a huge crack in it. So I was thinking of changing the rings (while I am there I mine as well) and dropping in a 3.1L crank and putting my cam and heads on it. If this is possibly punched out that big and I put that crank in it could anyone tell me what my displacment might be? I have no idea how to figure it out but I am sure some of you do. Or atleast could give me a geuss?? Does this sound like a good setup to you? Everything else including the cylnders pistons and rods are in great shape but I will have the block dipped and inspected just to be sure.

Anyhelp would be great!!
Thanks in advance!

Nick
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83' s10 extended cab 4x4 supercharged 2.8l v6
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post #2 of (permalink) Old 09-14-2000, 10:53 AM
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Re: 2.8 punched .75 over???

First off a .075 overbore is not reccomended for any 2.8 block ,less the aluminum bowtie one. .060 is the maximum recomendation , but I would only go .030 or .040 over. But your motor appears to be .075 over(gripes!)so I wouldnt use it unless your using low compression pistions (economy). If you wish to use the 3.1 crank to stroke the motor , then you will need 3.1 pistons. The only problem with stock 3.1 pistons is they dont yeild much compression in stock forms, so Custom pistons become the only sutibale choice for increased power out put. Iwould buy a new 2.8 block, I saw one dealership with em for around $400 , and the 3.1 crank is $300 new also,and get Ross to make you some forged pistons to make a better safer stronger 2.8.

post #3 of (permalink) Old 09-14-2000, 11:55 AM
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Re: 2.8 punched .75 over???

Actually I don't know what I was thinking last night, Because they are stamped .75 obviously this would be impossible. I thought that a bore of .75 would be much. I will get out the inside calipers tonight and measure it to see if it is bored at all. I am just wondering why you would have to use the 3.1 pistons? Do you know if the journuls(sp?) are the same size on the 3.1 and the 2.8? I am kinda looking to lower my compression as it is, I think I am running too much right now with the supercharger. Do you know any reasons that the piston would be stamped .75? I ran this motor for about 2 years after I bought it and never had a single problem and it had great power till the crank decided it had enough. thanks for your help


Nick
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83' s10 extended cab 4x4 supercharged 2.8l v6
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post #4 of (permalink) Old 09-14-2000, 08:38 PM
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Re: 2.8 punched .75 over???

Im not sure what the .75 stamped into the tops of your pistons mean. Perhaps it was just an Id number.

I belive you need the pistons used in the 3.1 due to the wrist pin placement being different between the 3.1 and the 2.8. I could be wrong , but I do know that the 2.8 and 3.1 use the same 5.7 inch rods. The journals are exactly the same on a 2.8 and 3.1 and the 350 Smallblock.Also 350ci chevy rods of any type can be used if the Main jornal sides are machined narrower.

post #5 of (permalink) Old 09-14-2000, 09:15 PM
 
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Re: 2.8 punched .75 over???

An engine's displacement is simply the volume of all its cylinders:

"Pi" (3.14) x 1/2 the bore (squared) x the stroke x the # of cylinders the engine has.

For a stock 2.8L V-6..... bore = 3.50" stroke = 2.99"

3.14 x 1.75 x 1.75 x 2.99 x 6 = 173 cubic inches

To convert cubic inches to liters divide by 61.027
To convert liters to cubic inches multiply by 61.027.


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post #6 of (permalink) Old 09-14-2000, 09:49 PM
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Re: 2.8 punched .75 over???

Ok But if the 3.1 and 2.8 use the same rods then couldn't I just use the 2.8 rods and pistons? They are in excelent condition and I would have the cylnders honed out and put new rings in. I am going to call a shop tomorow to get a price on the honeing and putting new cam bearings in. I am thinking of haveing them setting in the crank and pistons just cause I have only done a few and they were no high performance motors. Can anyone give me a geuss of what it might run to have this done? just wondering before I get to calling around. Thanks for all your help so far!! I will try to figure my displacement out tonight.

Nick
[email protected]
83' s10 extended cab 4x4 supercharged 2.8l v6
post #7 of (permalink) Old 09-15-2000, 07:50 PM
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Re: 2.8 punched .75 over???

Hey nick, ask the machine shop if you can use the 3.1 pistons, and by all means let me know wasup! Also Id estimate $90 on ck-10 honing,40 on cam bearings, and $ 150 for assembly. Hope to hear back! My 2.8 s

post #8 of (permalink) Old 09-16-2000, 12:09 AM
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Re: 2.8 punched .75 over???

I just read on the GM hi po catolog that the 3.1 crank in an 2.8 causes the piston compression height to be too high due too the longer stroke. I knew it. So yea ya need the 3.1 pistons

post #9 of (permalink) Old 09-16-2000, 12:40 AM
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Re: 2.8 punched .75 over???

the .75 on the pistons is a over bore measured in millimeters this equals .20 of a inch. .75 in inches is 3/4 of a inch and yes that is far from possible in any engine that I am aware of.

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post #10 of (permalink) Old 09-16-2000, 11:23 AM
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Re: 2.8 punched .75 over???

There is a company selling a 3.1 stroker kit for the 2.8 on eBay, check it out:

http://cgi.ebay.com/ebaymotors/aw-cg...item=431842659

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