Has Anybody machined domes off pistons to Lower Compresion?

I have a Chevy 327 with TRW froged domed pistons and closed chamber heads. Pushing 12:1 compression I am looking for 9.5:1. If anybody has access to TRW part numbers from the early 70s I could get the part number. I would like to turn the down the domes on manual Lathe using a Digital readout and a 10 inch chuck. The machining part is not a problem for me. The question I have is there enough material left over on the piston to make it all worth while? Would it be as good as new? The other question is on CC's? Could a guy drop the piston in a large beaker and see what it displaces before and after machining to make sure the exact amount needed is removed? (no bubbles.)
I guess the beaker would have to have the proper graduations.
FYI I am a machinist not a Chemist.

I have access to the machine and I am trying to save a few bucks.
But I surely dont want to do anything stupid. Thanks.


Chiefton54

 

You'll never be able to remove enough material to drop the compression that far.

These are hollow-dome pistons, they will just become too thin to hold up if you try to remove too much.

You might achieve a 1-point reduction at best, but you'll never see a 2-1/2 point reduction.

My buddy tried the same thing with some TRW domed Pontiac pistons that were made back in the '70s, he was surprised at how little could be removed before they became too thin.

His 12:1 400 engine became an 11:1 engine after cutting the domes down, but it still ****ed because it wouldn't run on premium gas without backing off the ignition timing quite a bit and giving up a lot of power.

 

This should bring out the "no it all's"! I will be first to say I have done this very same thing for BB blower motor pistons. You do need to do your home work as to top thickness before and after machining. Lots of pistons have enough material to pull this off in your lathe, naturaly balancing afterwords. I have to ask what you are trying to gain other than save a buck or two?

 

Looks to me like somebody just want to play on his lathe......and ruin a set of pistons

 

Thanks for the reply. I havent torn down the motor yet. But that was exactly what I was affraid of. I quess I am in the market for new pistons.

 

I replied to soon. What I want to accomplish is not having to purchase
spendy 100 octane fuel and burn easily found 92 instead. Also be cheep
and play on the lathe.

 

Post the part numbers on top of pistons.................

 

Give me a few minutes to dig through receipts.

 

1690 I am ***uming they are TRW. Since everything else on that receipt is.

 

$18.00 dollars each in 1974. .030 over.

 

http://www.jalopyjournal.com/forum/showthread.php?t=36010

 

Thanks Brigrat!

 

Stupid question. Do I have to get everything rebalanced when replacing old with new pistons?

 

Yes, rebalance...............
I am not saying you should shave your pistons all I am saying is it's been done before and worked. All the ones I shaved were new domed pistons, yours are used and of unknown condition.

 

Do you have to re-ballance if you buy new rims to go on your old tires?

 

I am not going to risk it. I will purchase new ones. One less thing to worry about. What does it cost nowdays to balance everything?

 

A 327 is balanced with the crank out of the motor, is it not? So there's no point in balancing it because of the piston issue. I thought only SB 400s where done in the motor. Also, the domed pistons would probably bring a few bucks on Ebay or whatever and then you could buy flat top pistons. Why not save yourself the h***le?

 

Cost depends on who and where, $250.00 - $500
If your pistons are in good usable condition I may have a use for them if they are available.

 

327's are internally balanced. I have no clue as to how they do it.
If anybody has the time please school me.

 

The harmonic balancer is for hormonics, you still need to balance all parts if you want a smooth long lasting motor. Their are a few diff. ways to balance a motor, check around with the more popular machine shops around your area. Most balance the harmonic, crank, rods, pistons, rings, bearings, flywheel or flex plate and clutch if it's not a Center Force.
To balance they take away or add wieght to the crank, lighted rods to the lightest one, drill flywheel or add to flexplate, etc.

 

Not exactly.
The crank is ballanced with it out of the engine on both internally and externally ballanced engines. The difference being that on an externally ballanced motor, they [they being the machine shop] would ballance the crank with the flywheel and harmonic ballancer bolted to the crank.

Ballancing the remainder of the rotating ***embly means weighing all the pistons, and then removing material form the heavier ones untill they are all the same weight.

Once that is done, the rods are ballanced in a similar fashion, then,with the pistons installed, they are all ballanced untill each one is the exact same weight


A full on balancing job can get really crazy, going so far as to measure the tension pre-set of the rings, CC-ing the piston tops and combustion chambers, etc, etc., etc., in an attempt to make all 8 cylinders EXACTLY the same.

On a nice stock rebuild or a mild street engine, the builder will usually ballance the crank only.

On a hot street, or race engine, the entire rotating ***embly would be balanced. This includes crank, rods, and pistons.

By the time you get to NASCAR or INDY engines, they will go so far as to balance the valve springs, acessory pullies, distributor, even the belt on the front of the engine. It might sound crazy to spend thousands of dollars to free up only one or two horsepower, but at the end of a 500 mile race, that extra 1 or 2 HP might mean the difference between first place and loosing!

 

A full on balancing job can get really crazy, going so far as to measure the tension pre-set of the rings, CC-ing the piston tops and combustion chambers, etc, etc., etc., in an attempt to make all 8 cylinders EXACTLY the same.

And now you enter the world of bluepinting............<!-- / message --><!-- sig -->

 

I think brigrat has the right answer if you are going to do it.

 

Bigrat,You are correct up to a point..just failed to mention that a bobweight representing the rods,pistons,rings is bolted to the crank on the rod journals in order to represent the weight of those items.Bob weights are made up per the required weight that bolts to that journal.A.E. 2-rods 2-pistons ,rod brgs for 2 rods ,rings for 2 pistons ,2 wrist pins, =total weight of bobweight

 

Made many piston for experimental engines from TRW cores. Minimum thickness for TRW forged is .188 . As long as you have that it will be fine. You already KNOW the valve pockets are OK because they were made that way. If it is NOt a hollow dome piston then it will likeky be ok. How to measure piston dome thickness? Glad you asked. Your drill press has a ruler on the side of it. Put a blunt shaft in you drill press. At full depression of the quill make sure you have .188 between the end of that shaft and the table. Turn the piston upside down and using the quill check to make sure no area of the piston is LESS than that .188 distance.. Mopar 12.5 to ones for both 383 and 440 used to say in the TRW catalog "may be machined flush." I have done these several times to get a nice strong reasonably light 10:1 piston. You can get there fairly easy if the Chevy piston are similar in thickness and design. You may need a thicker gasket to squeek the 9.5 to 1 or a larger head (68 to 70cc ). Seems to me from memory 327 heads are 64 cc but dont take that at face value without checking. In fact i check everything and always have even torque values I have memorized because in the heat of the moment it is just too easy to make an error. Anyway if the piston dome is a lump of solid aluminum which it usually was back in the day then yes you can machine that off providing everything else I said is right. Minimum safe distance for crown above top ring groove is .150" but you will be way above that. You shouldnt have to machine below the flat part of the lower part of the piston anyway and that in fact would be a great reference point to stop at. I often use the top ring groove as a reference for machining. Cast (as they are called) pistons need .288 dome thickness. You will need to rebalance the motor afterwards but should anyway.
Don
Here is the last set I made. These were .030" 440 forged trw flat tops (10 to 1s)
They are now 400 .010" stroked to 426 12 to 1s. They have stood three years of hard racing at as high as 7800 RPM in an engine making around 640 to 650 horse. I know that isnt what you are making but i wanted you to see I wasnt just blowin smoke. Been there done that many many times. You can too.

 

The .188 would hold true for flat-top pistons if you cut a dish in them?

455 Buick TRW forged.

I wouldn't want to come that close, but its nice to know the minimum thickness on TRW Forged domed pistons.


Fwiw - PAW has a good selection of SBC pistons for reasonable prices.

 

I had my machinist take a little off the top of my TRW forged 302 chevy pistons...He charged me 7 bucks a piston and with 64 CC heads it brought my compression to 10 to one..he was balancing the recip ***embly anyway so I didn't have to pay for rebalancing..
TRW forgings do have a hollow dome so make sure you leave the .188 already suggested.

 

TRW used to publish the specs for machine shops back in the day. Minmum safe zones for much of the pistons areas. That is where I got the specs. Probably 25 years ago. I probably still have that book here somewhere but t would take a week to find it.

 

Have you thought about changing to heads with larger combustion chambers?

 

Part #s from 1984 TRW catalog for 327.
L2165F w/4 valve relief on head.
#L2166NF with .125 High step and valve reliefs on head.
The truck 327 lists #L176F with 2 valve reliefs.
I think the first 2 are forged and the truck pistons are Power
groove. Not positive on anything.
I do know some TRW flat top forged pistons are cut to lower compression for other chevys. Search octane cut.