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I am trying to price a Crank for my 327 block, I am asked if it has a 1-piece rear main seal or 2. Attached is a picture of the bottom side, how do I tell?
 

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Look at the back side of block
If a deep groove just inside the bearing cap, 2 piece.
If no groove and there is a bolt pattern symmetrical about the rear cap, then 1 piece
 

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Here is what a one piece rear main seal looks like from the back of the block:



and here is what a two piece rear main seal looks like:



Notice the rear main seal on the top photo is one big seal whereas the bottom there is no seal shown but it fits over the ridge in the back and could leak at the joint (which is why you off set the seal about fifteen degrees and use sealant).

Found a two piece seal photo. The seal is the blue bit.



Big Dave
 

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Thanks for the pictures, I will check it out. I was told by tech guy at JEGS today that the older blocks had the 2-piece and it wasn't until mid 80's did they switch to the 1-piece.
 

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Thanks for the pictures, I will check it out. I was told by tech guy at JEGS today that the older blocks had the 2-piece and it wasn't until mid 80's did they switch to the 1-piece.
GM started the conversion in 1985 and everything had a one piece rear main seal and hydraulic roller lifters by 1987. As such if you look in the lifter valley you will see the stand offs cast into the valley if it has a one piece rear main seal if those stand offs are drilled and tapped it was used as a hydraulic roller. The lifter bosses are also taller on a one piece block but that is hard to determine visually.

Big Dave
 

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Can you tell if it's a one-piece or two-piece seal by the dipstick location? I was told once that if the dipstick is on the driver's side, it's an older two-piece block; if it is on the passenger side, it's a newer one-piece block. I've always wondered if that is really true.
 

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It isn't.

Yes the block had a hole drilled in it to move the dip stick away from the oil pan (so that the actual hole to the outside world was raised above the splashed oil level in the pan), but it was moved from the pan to the block in the 1980 production run. It was done to relieve oil leaks, but ....

If when you have the motor rebuilt the machine shop will drive the lower part of the block mounted dip stick tube out of the block and they don't replace it. You have to buy a replacement tube from Chevy or the oil pressure will either blow the dip stick out onto the highway or just push it up to the hood and blow oil everywhere. Also without the lower tube in place the upper tube is a poor fit.

Back to the question: dip stick moved in 1980 and one piece rear main seal didn't appear in Camaros until I believe it was 1986 so it isn't a true indicator.

Big Dave
 

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Hey Dave. You say to offset the 2 piece 15* yet the picture you posted Ike's not show it offset?
 

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I don't think that is Dave in the photo. He just said he "found" a photo of the two piece. I agree, the seal needs to be slightly off set from the flange of the main cap and the block. Doesn't need much. Also, don't need near as much RTV as in that pic.
 

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Hey Dave. You say to offset the 2 piece 15* yet the picture you posted Ike's not show it offset?
That isn't me or my picture I quickly mined it off the web.

When I am building a motor I am usually paying attention to what I am doing and usually have oil covered hands or fingers with RTV or other sealants on them which I don't want on my Nikon camera or lenses.

My note books are nearly illegible for this reason. I write down all of the measurements and what I am doing as I often have two or three motors in one stage of assembly or another and didn't trust my memory even when I was younger.

This became especially true after I lost a 454 to a loose oil pump bolt that I didn't torque down after mocking everything up to check clearances. Finding the oil pump and bolt in the bottom of the oil pan reinforced my need for documentation.

Big Dave
 

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I was just funnin ya. Lol

I am a big fan of check lists due in part to my pilot training.

I also have a dry erase board on my garage wall that I write things on as I'm working, greasy hands and all. Basically I write down anything that I plan to come back to.
 
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