You got it!

Even at 15% (acceptable) leakdown, there will be slight air leakage going by the rings, so you will hear that light hissing sound by removing the oil filler cap from the valve cover. But leakage over 20% will have significant sound coming from the oil filler cap, and you will be able to feel the air blowing out of the valve cover holes (oil filler & PCV grommet).

Likewise, a bad intake valve will result in air hissing from the carb, and bad exhaust valves will result in air hissing out the exhaust pipes.

So you look at the % number to see if things are good or bad. If things are bad, you listen and feel for the air leakage to immediately determine whether you have ring problems or valve problems (or both...).

The most accurate results are achieved when the test is done on an engine that has been recently operated so that parts are not dead-cold, and oil has been "normally" distributed onto cylinder walls and rings. In Denis's case, his engine has been sitting unused for quite a while, so he would get a false indication of high leakdown if he just did the test on his engine without running it first. So it's important that he get it mounted up in his chassis, stick a radiator in front of it (with just water in it), hook up a fan belt to drive the waterpump, stick the fuel pump suction hose in a gas can, hook up some battery cables and hot-wire the ignition system. It should be quick and easy to get it running, let it warm up, then do the leakdown test to see what the real condition of the engine is. I'm betting he might be pleasantly surprised..


Lars

 

Well a correct match will have the same camshaft (same specifications) as the original and the same compression ratio producing the same or similar power. '87 and later blocks had a one piece rear main seal with a different crankshaft bolt pattern for a different flexplate. And at sometime the fuel pump casting boss was not bored and machined for a mechanical fuel pump as fuel injection was/became standard. There are other differences as the sbc evolved over the 60 some years of production and there are books written that identify all the differences.

In your application I wouldn't worry whether you have a 2 bolt or 4 bolt block. The difference is the number bolts that secure the crankshaft bearing caps and 4 bolts are considered stronger while these are used a lot in trucks and hi-po cars.

 

FWIW. Large reman operations do Not tear down a discrete motor and retain-reassemble all those parts as one discrete unit.
They tear down lots motors and all cranks go into a bin, all rods in another, all heads in another etc.
The contents of each bin are sorted for acceptance to proceed to either scrap or further inspection/machining etc.
Motors are assembled from bins of usable parts; rods may be dimensionally OK but not a match to its journal-mate.
Does that work? Yes. Is that sloppy? Yes. Do some folk not give a flip that it can't be bored again? Yes.
Or care if walls so thin they don't transfer heat or seal rings as well as they should? Yes. Or care that R-L heads don't have matching cast number? Yes.

A popular candidate to replace an L48 or other carbed sbc has been an L31 Vortec; but most OE production L31 do Not have a finished fuel pump mount pad. Finishing required an additional operation step by installer/machinist.

Current production L31 replacements are now supplied with a finished pad; pump is no longer an issue for DIYer. Plus All new L31 have forged steel cranks.

FWIW, Some years ago, I bought and installed an ATK motor. A Mazda 13B rotary. It soon sh7t the bed; Apex Seals. Warranty denied and any legal action would have been ONLY in ATK's jurisdiction; thousands of miles distant. I ate that bitter pill. But, as a result, I did learn to build rotaries as well.

JMO So, if you want an ATK or its ilk, go right ahead; your car, time $ money.
But GM has a footprint in nearly every US/Canadian county/parish/province/jurisdiction. And for not a whole lot more $, an ALL NEW GM crate is money well spent YMMV

OP Denis you can use earlier or later sbc. Also GM is presently offering rebates with some crate purchases.

 

I would like to add that it does matter where the piston is and you want it at TDC on every cylinder you test. Why? Because bores wear in a "cone", the widest being at the top.....and this is where the piston is when the plug ignites. You want it in the spot with the most potential for leaking and where the most cylinder pressure is. I hold the crank bolt with a breaker bar when doing it.....

Jebby

 

Jebby -
I have found that if there is so much wear on the cylinders that they have any significant wall taper (which will also result in ring wear), that the engine will fail leakdown test regardless of the piston position.

Lars

 

True....point taken......just some of that "Good Practice" stuff that won't go away.....

Jebby

 

It goes on and on. You keep trying to bad mouth ATK but more than likely have never been there to see how they reman. You have never seen how they select parts but make negative assumptions. They are an ISO9000 company which means all their machines meet international standards for calibration. How many local rebuild and machine shops can say that?

The rotary motor cases have a finite wear life as most rotary mechanics are aware of that and I have owned one myself in an RX7. If the motor and cases last for 90K miles you got your moneys worth.

Now you want to talk about using a L31 for replacement. That's an one piece rear main seal block with vortec heads so his flexplate won't fit nor will his intake manifold. Maybe the exh manifolds will work and so what if it has a forged crank. He doesn't need one currently. Your just confusing the OP. The OP doesn't know the difference between a 2 bolt or 4 bolt block. Do you actually think your helping him???

 

Jebby -
Yeah, I catch myself over-doing precision and procedures, too... tough habit to "moderate"...
The other "rationale" for just doing the test at bottom center with valves closed (in addition to not having to restrain the engine from turning over) is that you still get valid "comparative data" between cylinders: As long as all cylinders are measured the same way (pistons all in the same position from bore-to-bore during test), you can compare one reading to another to get a good "health indicator" of the engine. If you have 6 cylinders leaking at 12-15%, and see one at 22% and another at 25%, you know you need to fix a problem regardless of where the piston position was... If Denis gets results like that, it's time to start shopping for a crate... At that point he can start getting smart about 2-bolt versus 4-bolt; forged cranks; cam specs; centerbolt valve covers; fuel pump mounting pads; 2-piece rear mains; and all the other fun stuff to learn about...

Lars

 

veg-o-matic vege VC42
https://lkq.com.mx/galeria/

http://atkvege.com/wp-content/upload...engine2016.pdf

 

I have a 77 just run it ,mine was low and it still ran like a raped ape No gains throwing money at it unless you are shooting for NCRS.If it is going tobe your daily driver yes bite the bullet and get a new GM crate but who knows who is putting them together and with a lot of out sourcing ( Chinese parts) the low bidder gets the job in todays world.

 

Ret'd Process Eng, Q Eng, Q Mgr (ASQ/ASQC) have led Tier 1 & Tier 2 manufacturing companies to attain & maintain AS 9000, QS 9000, ISO 9000, TS 16949 & haver been on floor of Many other manufacturers, I don't know it all nor do I know what you do. But I've put in the work. If you're happy w/ ATK & its 4.060" bores; hurrah! I'm not.

cows still ain't come home.

 

Denis-
Disregard all the "noise" in your thread here. Get your engine installed, fired up, and leak-down tested. Then worry about making a decision on how to proceed once you have the results/data. If you need assistance in hooking up your engine to fire it up in the chassis, let me know - you can e-mail me directly if you need assistance without unnecessary distractors. Jebby is also a good technical resource.

Lars
V8FastCars@msn.com

 

Close for review.

Re-opening.

Guys, this thread has good information in it. I’d like to save it. If the hostilities resume, the vacations will get longer.

 

Fair enough vettebuyer

As an example:
ATK HP99C does have 4.060" bores and ATK publishes that on their site under tech specs tab (below). Some others are +40 while others +60.
https://www.high-performance-engines...hp-p/hp99c.htm

FWIW, BluePrint (as well as others) does likewise; understandable as good cores are becoming increasingly scarce. That'll only continue.
Which, IMO, is all the more reason to buy a NEW motor that's not already so worn; I recommend new motors from GMPowertrain/GM/Chev.

OP Denis
I hope your 77 motor is fine after its storage and I hope your leakdown checks out AOK and hope you won't need more repairs or another motor.
But if you or others need a replacement motor, I recommend a NEW Motor from GM/Chev as opposed to any Reman; as most Remans have the cheapest, less than ideal parts and internal dimensioning.

 

I have one final question on this subject. If you have REALLY good compression numbers (top of the expected range) on all cylinders from the simple compression test, and yet still burning oil; why the need for leak down test? Seems like the issue would have to be with the heads. How else could oil be getting in the combustion chamber if it's not coming from below (past the rings)? I understand that there must be a reason and I'm showing my ignorance, but don't care/still trying to learn. Thanks in advance, EC

 

THIS is what ive done with my 74... although instead of buying a crate engine I decided to build my own 355 for a purchased bored over 4 bolt block and built the entire engine carb to pan for under $1500 with some mild performance parts such as used dart heads, flat top pistons and a voodoo cam... reliable but with more power than stock. I also had no idea how to build an engine so asked a lot of questions here and watched a lot of youtube videos.