Hi everyone,
I have been working on an upgrade to my 1972 L48 matching numbers engine which includes vortec heads, new roller cam, roller lifters, intake, headers, and exhaust.
I've got the engine pretty far disassembled, with the heads off, and have been cleaning the deck and block as much as possible. Now that I can see into the cylinders, it looks like there is a decent amount of pitting on cylinder #7, and a little bit on cylinder #4.
From what I've read, many times pitting comes from gasket leak, leading to coolant going into the cylinder for extended periods of time. From my novice view, apart from the pitting, the cylinders don't appear to have much if any cross hatching left, and look pretty polished. Although, before disassembling the engine, each cylinder made pretty good compression at about 150 psi, and oil pressure was good, at about 45 psi at warm idle. The engine leaks, and burns oil, at about 1 quart every 500 miles. I'm not sure how much is being burnt vs leaked out. I looked at the plugs, and I'll admit I'm not very good at reading plugs, but I would say they look oily.
I was trying to avoid pulling the engine; I have never done that and fear the pain of putting it back into the car correctly.

I'm leaning toward pulling the engine and getting it bored out, since I'm going through it all right now, but I'm not sure what replacement pistons to get. I think the motor should make about 330hp at the crank (mild cam since I have auto with 3.08 rear) with the cam I've selected, so I don't think forged is necessary. I am confused what piston compression height to get; I see 1.540 to 1.562, but not sure what compression height the stock motor had. Since the vortec heads have the smaller combustion chamber, 64cc, and I was planning to use the 0.015 head gasket, I don't think flat tops are a good idea. Should be about 9.5:1 when done. I see most dished pistons with 4 valve reliefs are about 12.3 cc dish, but not sure how that works with the piston compression height.

What do you all think?
Would you pull the engine, or would you just button it back up to see how much oil the engine is actually burning vs leaking after new valve seals and various gaskets?
Has anyone replaced the stock pistons successfully? Which ones did you use?

 

 

I think you already answered you own question ,, your half way there
I will be waiting to see what everyone says .. in the mean time I would mic the cylinders from top to bottom and see how far they are out

 

Almost looks like there was detonation going on in there and the walls don't look good either. How were the plugs, did they have metallic specs on them? For the brand, I would go with Wiseco forged, but that's just me. They are good pistons and you can get them in any size you want without spending a fortune in pistons. I would let your machinist work out the details on how to go with the machining of the block and then discuss it if you are not happy with the numbers he provides you with. A 10.1 motor would be nice, but if you want 9 or 9.5 then do it.

 

I definitely would.

 

Thanks for your response.
With the block only being a 2 bolt main and cast crank , I'm leaning more toward the hypereutectic kind. This car will not see any racing or high rpm, just a cruiser.
When I pulled the plugs I didnt see any metal particles on them, or in the oil pan when i removed it. Detonation is definitely possible, though; when I bought the car the distributor was all gummed up, but I would think that would result in not enough advance rather than detonation.
When you replaced yours with the Wiseco pistons, do you remember what compression height they were?
Can I assume you installed flat top with 4 valve reliefs?
With the vortec heads being iron, I'm worried 10:1 would be too much, especially since my cam doesn't have a lot of duration/ overlap.

 

You have a very good plan for a very good combo.

The bore looks awful. It won't last long with more compression. Have it bored & fitted with new pistons. First have the machine shop tell you MINIMUM that will clean it up; first oversize for sbc is +0.020" & hopefully that's all it'll need.

 

So here are the plugs

 

More plugs

on the bright side, cylinder #2 looks really good. Ha

Before disassembly I bought a newer Q-jet and was getting it tuned as close as possible to ensure a good cam break in. That's probably contributed to the sooty plugs. Since vortec heads don't have heat cross over, had to get a model with electric choke. The newer model had transfer slots, which I believe gives it more kick, or maybe I just got it more dialed in.
I like the divorced choke better though

 

Any advice to removing the kick down connector on the TH400?
It's wiggly, but doesn't want cone out. Only turns about 1/4 turn.
How are they intended to disengage?

 

Great advice right there. You don't need to go overboard with pistons for the rebuild for you're planning but once the shop gets you to a solid starting point you will definitely enjoy the new power with the cam/heads and have a piece of mind that you can drive it like that for many miles since it was done right. There will definitely be some added costs but the bright side is there are tons of SBC options and the cost is cheap compared to other engines.

Let us know how it goes.

 

Before I committed to boring the block and the new pistons, I'd check the taper and out of round on all 8. Then figure out if it's more economic to bore and new pistons or another block.
It may be that a good hone will clean everything up.
I'd also look at the cost difference between the hypereutectic and forged, remembering that forged will take some detonation with out self destructing. And you're in there, check the crank dimensions- If things are worn to the point the crank needs turned, it may be that it'll be less cash to build a 383.
I'll admit up front I'm not a big fan of the hypereutectic pistons in applications that are not computer controlled. Too easy to make a mistake and go into the detonation zone.

 

The connector on the 400 is just like to old oil pressure connectors- the male part that comes out of the trans has a lip and the rubber female side snaps over it. Try hooking it gently with a pick and working the lip off. Heat and oil makes the old rubber difficult to work with

 

I personally do not like the Hypereutectic pistons. I had a set of KB pistons once that went boom after detonation, they just would not hold up. I was pushing the motor to the limit and it is my fault, but forged pistons can take more without issues normally. If you are just building a cruise motor, than I guess you would be fine. Here is what I put in my new motor. Wiseco pistons Mine are 10.5, but if your using iron heads you might not want to do that and 9 or 9.5 would be fine.

 

 

I agree with the others, those bores are done! That pitting needs a bore to get rid of it.
Most stock engines have the pistons at least .020" down in the bore, so getting the max compression height is good. I'd go with low expansion forged pistons.....the extra $300 spent now will provide comfort down the road if you want more.
Here are some nice SRP's for under $550

https://www.summitracing.com/int/par...make/chevrolet

 

Thanks for all the advice!
I finally got the kick down connector out after much fiddling. Hope I didn't break anything.
I have almost everything ready for engine removal, apart from getting a hoist and doing it.

...still have to figure out what piston compression height to get, as the 1.560 seem to be something special because when I put in 3.48 as my stroke only 1.548 seems to come up.
does 1.548 seem right for stock?

 

thanks for the advice!
this is a matching numbers block, so I really want to keep it, even if it is a L48. I'm hoping this is sound logic as far as resale value. it's the standard 4.0 inch bore right now, so I'm hoping it shouldn't be a problem cleaning it up.
if it wasn't #s matching...well, let's just say I've been looking at used 454 a lot on Craigslist.

 

The differences you're seeing on compression height is to allow for decking the block. Many lower cost cast and hypereutectic pistons are made "short"..so the piston will remain down in the hole after decking. Fast rebuilders just want to cut everything to one measurement and know that whatever they put in there will run. Is it the right way? No. Often folks end up with pistons down in the hole .040-.060 and it just kills compression and likely to detonate worse. Most shops deck them to provide .005" or so deck clearance.....but since you probably won't be decking it...shoot for the tallest piston. Do the math.

You've got a 3.48" stroke. Divide by two.....and you get 1.74". Now add your 5.7" rod to it and you have 7.44". Adding the 1.562" compression height gets you 9.002".

"Nominal" deck height for a SBC is 9.025". So if using the 1.562" piston you should end up with around .023": deck height. But that assumes your block is 9.025"..which it may or may not be. As you can see that 1.540" piston would put you at .045" down in the hole at the best. Those are used for severely cut blocks.

I would suggest taking the block to the machine shop..let him measure it to determine how much it will have to be bored. And since it's a mild build...just let him get the pistons he trusts....but as mentioned I'd avoid the KB's for sure and I'm not a hyper fan either...but they will probably be OK for you...just keep it tuned well.

JIM

 

If you're not planning to go all-out, a shop with a machinist that knows what he's doing could probably hone to the next size. The bores may not end up quite as straight as if it were bored and honed, but they're pretty dang good, might save you some bucks. I've had it done and ended up with a good mill that didn't burn a drop of oil and ran fine.
It looks like it had water in it and was just cleaned up and assembled, the rust in those bores was a lot worse than you can see now. I've torn down blocks that had bores that look like yours and some of the pistons had to be hammered out.

I rebuilt the 350 in my '97 Tahoe with true flat top/4 valve relief hypereutectic pistons. Block was zero decked. Compression is at 10.1:1 with Vortec heads. I'm lucky in that I have a computer to control this engine but have never had any rattle that I could hear. Runs great with an LT4 roller cam. Last road trip in my 4200lbs rig got just shy of 18mpg.