377Z

iTrader: (1)

What is the correct way to measure piston depth? Everything I’ve run across says to just lay a straightedge over the hole and use feeler gauges, but where on the piston exactly? Is it in the very center, so its along the pin centerline, or towards the side the head’s “quench area” is? I had a minute yesterday, but couldn’t find my feeler gauges, so I just used the depth gauge portion of a set of calipers on the front and rear of each piston along the pin centerline. The problem is I got different numbers at the front and back of each piston. The best one only had a difference of 0.001” front to rear, while the worst two where 0.015.” The rotating assembly spins in the block fine, without excessive force required & doesn’t hang up anywhere, can this be right? Has anyone seen this before?

**edit** Pistons are forged flattops, rods reconned.

Streetiron85

You'd want to measure it above the wristpin, there's no rocking effecting the depth at that point.
A depth ga on a caliper ca be used, but there's a possibility for error if you arent careful about keeping it perpendicular to the deck.
Recon'd rods can sometimes have a variation in length of .010", depending upon how many times they've been recon'd or how carefully it was done. They get shorter each time it's done.
I'd say double check your measurements, and compare depth between the front and back on any given piston.

377Z

iTrader: (1)

Okay, so along the pin centerline is correct; but whats with the slope? I had checked piston depth at the center using a straightedge and feelers before I moved, and when I ran across random different "in the hole" dimensions from hole to hole. I couldn't get to this stuff for a while (until recently), so I decided to start over with the measurements. I had a spare minute so I figured I'd get ballpark measurements with the caliper to go run some CR/DCR calcs to get an idea of where I'm at. I measured above the pin at each end, figuring I'd probably see no more than a few thou difference front to back. Well, that was true for half the cylinders, but the other half have 0.010-0.015" variation front to back. I know stuff like if the block is out of square a bit, if the bores aren't perfectly square, slight piston manufacturing variations, etc. could come into play, but I think something like 0.015" seems kind of much. I don't think the piston could be coc ked this much; if the piston top is square with its skirt it couldn't **** this much. I would also guess its not something most guys would look for.

In any case I'm pretty fed up with this; I bought this stuff when I knew even less than I do now. I'm thinking I have no choice but to disassemble the whole deal, have everything inspected because my 'simple 383' has been a comedy of (my) errors so far, and the "f"ing jokes on me. I might just end up getting a budget steel 350 crank and rods from eagle, have the block zero decked for the new stuff, and screw the damn 383 because from this point it's not necessary to achieve my goals for the car and will only give me more headaches. This way, I'll have a properly built engine thats still 93% as big and will actually be built properly. Hopefully I can get a couple dollars for the unused stuff on ebay or something, maybe I can sell the crank and rods cheap to someone who has diluded themselves into thinking a 334 is a good idea.

Sorry for the rant; I hate wasting money, despite being good enough at it to make a r i c e r blush.

Streetiron85

It's obvious that if your measurements are accurate, there's an error somewhere in the machining.
Either the pin bores in the rods or in the pistons are out of alignment.
.010"-.015" difference from front to back in one cyl is a lot.
Be sure your measurements are accurate, and if they are, try to locate the problem.
What kind of pistons do you have?


TIP:
Fastening your vernier caliper to a square (I use hot glue) that will provide a stable perpindicular base is a good way to get repeatable readings that are accurate enough for your purposes.

F-BIRD'88

Get a long steel straight edge and check the deck surface of the offending bank for straightness.

8Mike9

Ya know, that's one thing I've never worried about, but brings some things to mind. I've always relied on the manufacturers spec on CR vs deck hiegth and left it at that.



Now I gotta another thing to look at and wonder. Thank you, IO now have to pull the head off th L-6 I just put together...naw, I'll let'er run the way it is.

377Z

iTrader: (1)

Thank you for the advice, guys. One thing is certain, I need to look at this again. My measurements definitely have some inaccuracy in them due to using calipers, but I'll check it again with feelers and a straightedge (or use the calipers/hot glue/square idea, have to see if the hot glue gun still works), as well as verifying the flatness of the decks.

Pistons are TRW L2256F's. I suspect the recon rods are the problem, if not at least a significant contributor.

Even if everything somehow checked out I wouldn't be comfortable running this thing as is, what with the rods being all different lengths and all. If the pistons check out fine, I'd like to use them, so if I want to stay 383 I'll get another set of 5.565 rods. New 5.565 rods seem harder to come by (makes sense). Eagle has some $499 h-beams (as well as some outfit named CAT out in CA, but I've never heard of them), and if I got these I couldn't see keeping the Chinese crank, so there is another $400-600. Otherwise, its back to 350 with a set of PM rods and the $400-600 crank again. New PM rods are $250-300, so I'd be paying between $100-250 to gain 27 cubic inches, depending on parts selection. Machining costs seem like they'd be equal either way. I'm not sure if I'll have issues with clearance with the 383, and I'm not sure its worth it for so few extra inches--looks like I have a decision to make, and parts to inspect.