HA! Brought another one back over to the dark side!
The low buck route is pretty hard to say no to once you already have the 4.2 crank and rods in hand.
Be sure to save the old 4.2 harmonic balancer so you can use it to make a spacer for the crank nose. Unless you got lucky and already found one with the short snout for the serpentine belt setup. What year is the 4.2 crank from?
the newer 4wt cranks have a 54mm snout where the harmonic balancer mounts - just like the stock 4.0 crank. the 12wt cranks have a 64mm snout. you can either buy a spacer from Hesco to solve the mounting issue, OR some take an old hb and cut a 10mm section off to make their own collar. Both ways work just fine!
ftpiercecracker1 wrote:its a 79 crank.
but i dont understand what saving the old harmonic balancer is going to help with?
Yep, you'll have the 64mm long nose crank then. Late model 4.2's with the serp belts (88-90?) and pretty much all 4.0's have a crank snout that measures 54mm. The the old balancer fits the crank nose (and the keyway) so just cut off a piece of the center 10mm thick. You use it for a spacer in front of your 4.0 harmonic balancer so the balancer gets pushed all the way onto the crank once the bolt is tightened down. That way your serp belt lines up and the balancer rides where it's supposed to inside the front timing cover seal.
Or, just buy a spacer from Hesco for about $30. Just trying to save you a few bucks.
Thanks for the helpfull hint, but how do I go about cutting a piece from the HB? I dont have a torch, but I do have a grinder. But, wont the spacer have to perfectly round/balanced too?
I used an angle grinder with a thin 4.5" cut off wheel to cut the center free from the three spokes. I'm going to turn the outside diameter down on my lathe to clean it up, but if you don't have access to one of those it can also be done with a regular grinder. Once you get it all round on the outside, you just need to cut a 10mm legnth off from it. Be sure to cut it slightly long so you can use either some large files or a belt sander to get it smothed out by the time you get down to the finished size. You'll want to keep the front and back edges parallel so it won't bind up on installation.
Another option some people do is to just stack up thick washers or use a piece of heavy pipe. With either of those options you'll have to cut a slot on the inner diameter to clear the woodruf key.
thanks for the suggestion, i will have to take a look at it when i get home. I dont remember seeing any "spokes", but then again i didnt look real close.
Also.
Would it be ok to go ahead and buy the pistons, Before the block is machined or should I wait until its bored? I know what bore I want, but I keep worrying that the pin height will be off and i will have to screw around with returning them.
If you forgot these are them (http://www.summitracing.com/parts/SLP-677CP30/)
4.2 rods
stock bore (3.875)
+ .030 will yield a 3.905
i figure, i order the pistons and tell the machine shop what the bore is and they adjust accordingly.
Sorry I didn't explain the spacer deal real well. I had to rush out the door to go take some mid term exams.
There may be a big enough section on the front of the HB you can get the 10mm piece from without not having to turn/grind the OD, but I'm not sure. The center piece I hacked out is out in the garage somewhere right now and I don't have a measurement for that yet. I'll know for sure when I get ready to turn it on the lathe.
As far as the getting the pistons first? YES, you definately want the shop to have them in hand BEFORE they bore and hone the holes. It's too risky to go off of the advertised piston dimensions. They will have production line tolerances (+/- how much I don't know) so you'll want the actual pistons you'll be using in your engine to take the measurements from directly.
ftpiercecracker1 wrote:so the shop should be able to account for the pin height?
Piston choice needs to be matched to the rods you use.....you use different pistons on 4.2 rods than you would with 4.0 rods.
The machine shop isn't going to be able to make changes to the pin height unless you get special rods with offset bushings and full-floating pins. $$$$
You can have the block cut (decked) to change the compression ratio. Maybe that's what you meant?
Walt K
Eastern Pa
2001 Cherokee 4.6 stroker 90 day build
Buick GS's and Saab turbos for other days...
I know about decking. I was just told by reliable resource that you MUST buy the pistons before having the block milled, so that the machinist can account for the slight variations in each piston diameter.
Ex.
piston #1. 3.9052
piston #2. 3.9056
piston #3. 3.9051
and so on and so forth, for the very best fit the machinist must have the pistons right infront of him so that each bore is exactly what it needs to be. I was also recommended to let my shop pick out my pistons A. because they get discounts and B. because they actually know what the hell they are doing.
