Most of this info comes from literature I have. Haven't found a web site with "tell all" info. The leak down test uses what I know is called "differential compression". There are 2 guages in the set, one with regulator for controlling of input pressure and the other reads the blowby or leakage. The piston is on TDC, input 80psi and read the leakage on the other guage. Rule of thumb for this is no more than 20% less than input pressure. I don't like using this method due to safety reasons. You're going to have to hold the crankshaft while doing this and if you're not on perfect TDC, the crank will spin from the air pressure.
As far as it holding up for what you are going to do with it, who knows. But if you don't what to pull the engine again, while you are this far with it, I would pull the heads and pan, remove the piston and rods, measure piston to wall clearance. If good, clean pistons, install new rings, flexhone the cylinder walls and remove ridge at top of cylinder walls. Check rod journals for nicks and scoring, if good, polish with light (600grit) sandpaper, measure and install appropriat bearings. Do the same with the main crank journals. And while you're at it, throw on a new timing chain and water pump. This is all good cheap insurance.

 

Now that I keep thinking about it, I might go for the 393 stroker kit that I have been researching. I might have to wait longer to get all the parts, but at least I know they are new and will hold up. The crank is only 264, the rods and pistons another 200 each maybe. I might have to upgrade heads though....anyone need a set of ported gt40p's? If I do something like this do I have to get the rotating assembly balanced somewhere? Thanks again.

Ryan

 

I would use the heads with the stroker. You'll need to port the other heads to make use of the extra cubes you'll be drawing in. That's what I'm doing to mine. If you're going to stroke it, definetly have the whole rotating assy balanced, not just the crank.
If you still want to dispose of the heads, I might be interested.

 

Well the local machine shop guy told me that the exhaust valves on these heads are the most restrictive part. He was going to do a 3 angle valve job, flatten the spring seat for double springs, and drop the valve guide 100 thousands for under 200. He told me if he was too enlarge the exhaust port it would be in the 300-400 dollar range. I have the setup right on my desktop dyno. Just by switching from the gt40p heads to edelbrock victors with larger valves it gains like 50 hp or something crazy like that. Im not sure what to do. I feel like the 393 would be restrictd by the gt40ps. What do you think?

 

The alternate way to get the same effect, if you can plug this into your dyno, is to change the lift and duration of the valves. It would probably be cheaper to get a custom grind on a cam than to buy more heads. I would call CompCams or Crane and tell them what setup you have. You really don't want to go higher than .500 on lift unless you go into the higher RPM range. But you can change duration to get desired effects as would larger valves.

 

You can go to a higher lift than .500 cam on a stroker motor of this size, CompCam will help you out on this. A big stroker kinda uses up alot of those smaller cams.

Same on the cylinder heads, if your going to have that much displacement, save your nickles and get a set of AFR heads, you'll get more flow out of the box with these than your going to get out of the home ported GT40's. The GT40s stock are flowing 196in/139ex at .500. A out of the box set of AFR 185's are 267in/185ex at .500. You could port the AFR's to get even more getty-up out of em with the 393.

My 408 is using CompCam 35-318-8, .555 lift on the intake and .565 on the exhaust. Desktop est HP 440, est torque 480 at 3500, 400 down at 2000rpm. Check it out in my gallery.