Machine shop advice

[Greg L.]

In choosing a machine shop to bore a block what would be good questions to ask before hand to see if they are up to par with this task?

I know boring shouldn't be big deal for even the most incompetent shops but I just want to make sure that I pick a competent shop and get a quality job.

The block is a 69 L36 and I'm putting in stock compression KB .020 oversize pistons that run at .002-.0025 wall clearance. They will also be changing the pistons on the rods for me as well.

Thanks,

Greg Linton
#45455

[Duke W.]

Re: Machine shop advice

The first thing you should confirm is that they won't deck the block!

There are two types of boring equipment - those that index off the block deck and those than index off the crankshaft axis. What type do they use?

Shops that use equipment that indexes off the deck like to "deck the block" to be sure it is parallel to the crankshaft axis. They rarely bother to measure to determine if this is the case or not.

This is why I always recommend checking deck clearance of all eight cylinders prior to disassembly. This will tell you if the decks are parallel and get and handle on managing final assembled compression ratio.

Beyond this the bores should be honed to achieve the piston mfgs. recommended clearance with stones suitable for a moly faced ring sets, which is the best type to use for a road engine.

Duke

[Greg L.]

Re: Machine shop advice

Thanks Duke,

I didn't know of the two ways to index the block, just assumed that it would always be off of the crank index. I was very straight up and to the point with the one shop that I talk to about not touching the pad and they understood so they must index off of the crank but I will ask to make sure.

What type of honing stones should be used with moly rings?

I also read someplace that as long as the replacement pistons weigh within 10% of the originals then there is no need to have anything rebalanced...does this sound right? I just don't really want to throw money out for a balance if I don't need to. If I need to get it balanced to make it as good as it was back in 69 then that's okay but to balance it just to make it "pretty inside"...well, I can't really justify that.

[Joe C.]

Re: Machine shop advice

Greg,

I would also suggest that the cylinders be bored and honed with deck torque plates in place. The cylinder walls will actually strain up to .004" because of stresses imposed by the head bolts. The torque plate bore/hone will ensure perfect symmetrical bores for better sealing and oil control, lower friction, less piston slap.
Be sure that the machinist uses the same head bolt type that you will use to install your heads, torques them to the same value that you intend to, uses the same type head gasket for his plate, and uses the same type of thread sealant/lubricant that you intend to use.

Joe

[Mark #28455]

Balance it!

Make sure to use the torque plates when honing. You should get it re-balanced (about $100 at the shop here). In addition, given the cheap price of the Eagle and Scat rods ($200 to $250 a set) compared to the price of new rod bolts ($50 plus for ARP Wave Locs) plus labor ($100) plus pressing off the old pistons, etc, you should probably get the new rods.

Just my 2 cents, but it's a real pain to keep pulling the engine to fix it later. For less than $100 to $150 difference, just do it as best as possible the first time. I once followed the advice you were given with a street 454 I had rebuilt - only swapped pistons, no rebuild of the rods/crank - yes, it had a noticeable harmonic at just shy of 3000 RPM after the rebuild. Now I know better!

Good luck,
Mark

[Duke W.]

Re: Machine shop advice

I think the final hone should be with 600 grit stones and an automatic hone is the best machine. Ask the shop to go over the process with you.

The other advice is good. Get the engine precision balanced. Don't cut corners. It's not worth it.

Duke

[Larry L.]

Re: Machine shop advice

Follow the advice and you will not be disapointed.
Insist on the torque plate 2" thick minimum and when you look at the bores you can see the distortion in the hone patern.

The balance is worth every penney.
Some misguided shops will drill holes in the back side of the piston dome to match the weight - never do it. Take the metal to make a set of matched pistons from the skirt bottom as there is no stress here. If one piston is way out of line reject the set - get another.
Independently balance the vibration damper, fly wheel and then the fly wheel and clutch assembly.
In cae you have to replace something.
Inspect the vibration damper - and if there is any question replace it with a new part.

[Mark #28455]

Any hammered-on dampener should be tossed! *NM*

[Greg L.]

Re: Machine shop advice

Thanks guys thats all great advice.

Now about balancing.... What all needs to get balanced because of the pistons change and what should get balanced for a production balance job? My understanding was that the 427 was internaly balanced so that would mean that the flywheel or flexplate along with the harmonic dampener would not be balanced with the engine...or was it. I also found a shop that balances the old way by spinning the assy to high rpm. I call this the old way because he says that a lot of the newer shops only spin it up to a much lower rpm and rely on the computer to figure out the balance. Is one way prefered over the other or are they both good?

[Duke W.]

Re: Machine shop advice

The eight piston/pin assemblies should be balanced to within a range of one gram. Same for each end of the conn rods. The shop will need one ring set and one rod bearing to weigh.

Once the above masses are known the crankshaft is mounted in the machine and proper bob weights to represent the rotating mass are attached to the journals. The crankshaft is then spun and the end counterweights are adjusted as required to balance the total rotating mass and the first order rocking couple that is generated by the reciprocating mass. It's equivalent to dynamically balancing a tire with weights on each side.

The rotational speed required is a function of the equipment and newer equipment probably doesn't require as high a speed as old equipment. I would go with the shop that has the newest equipment, and ask them to explain the process. A modern "computer" system is probably better than some relic from the pre-computer era.

427s are internally balanced, so the flywheel and torsional damper (There's no such thing as a "dampener".) should be static balanced to the accuracy of the equipment. Ask what it is. My guess would be about 5-10 cm-grams.

Then the clutch assembly should be bolted to the flywheel and the entire flywheel-clutch assembly balanced with mass added or subtracted as necessary from the clutch cover. The flywheel and clutch cover should then be stamped to show the assembly indexing. OE flywheels and clutches have a stamped "X" to indicate correct indexing. Replacement clutches do not, so locate the flywheel "X" and make sure the shop stamps an "X" on the clutch cover next to the "X" on the flywheel.

The above type of precision balancing should result in a very smooth engine. Many whose engines have been so balanced report that the annoying "shifter buzz" common to vintage Corvettes disappears. And it's a good idea to get the driveshaft balanced, too.

Duke

[Greg L.]

Re: Machine shop advice

I was just thinking...do the main bearing caps have to be installed and torqued to specs for any of this work that I'll be getting done?

[Mark #28455]

yes *NM*