Trying to get somewhat educated before I talk to the machine shop. Can/will honing help straighten/round these cylinders when opening them up 4-5 thou for increased piston-wall clearance (for forged pistons), or does honing mostly follow present contours? '66 401, cylinders are in spec per Buick manual. Out of round is typically .0015", worst is .0025" Spec is <.003" Taper is typically under 0.002", worst is .0036" Spec is < 0.005" Cylinders were measured in 8 locations, pin and thrust sides at top, middle, and bottom of wear area, as well as unworn area at bottom. I used a bore gauge set up with 5" mic at 4.1875". Goal is to straighten out deviations in the cylinders to use std size TRW forged piston I've had on the self for years. They were likely manufactured in the 70's. Stock bore size is 4.1875", pistons measure 4.1862- 4.1870" @ pin. (.005- .0013" clearance). I was surprised to see the forged TRW's are near stock dia and not 0.005" undersize for clearance. I read these are VMS-75 alloy which is similar to 4032, which doesn't have a lot of heat expansion. I see recommendations (on Speed Talk) to go with at least 0.0045" clearance, and up to 0.006" piston-wall clearance before they get noisy. I'm not building a race car, just a cruiser than will likely not be driven 10k miles in the next decade.
I went .0035 with the Auto Tec 4032's, seems about right. Noisy during cold start, but quiet down within a couple minutes. And dont forget you can hone to whatever size you need and use a Line-2-Line coating to take up the slack (within .002-.003 I would think) My personal take on honing is it will mostly follow existing contours, but are you measuring with a torque plate or head installed?
Measurements are without a torque plate, heads not installed. This is for a 2nd 401 build. Original plan was to bore for the +0.020" cast pistons I have. But since the bore is within stock spec, figured I could use the std TRW's as this is likely the lowest-wear block I have. Also want to use up some of the parts I've collected over the years for 'someday' projects. I thought about coating the pistons if necessary. Have to see how the bores clean up. My primary build (silver 4 spd car) has more wear and will get the Auto-Tec's. Heads and rods are at the shop, so I'll soon have the rest of the needed measurements to finally order pistons from Tom.
People confuse honing with deglazing. Honing is done with long straight rigid stones set to a determined diameter that can eliminate/minimize irregularities'. Deglazing is done either with spring loaded straight stones or the christmas tree ball type deglazers that will mostly follow the existing irregularities' .
your trw pistons are most likely 2618 alloy you will need a minimum of 5.5 - 6 thousand clearance. on a 4032 alloy piston I would go at least 4 thousand. better a little bigger for scuffing when the engine starts running hot. on my 425 block with 4032 at .004 there was scuffing. then on the refresh we went closer to .005.
I don't positively know what the TRW forgings are made of. I've read VMS-75 according to Speed Talk guys. Either way, I'm going to need at least 4-5-6 thou to clean up the cylinders. I have forged TRW's in the 425 in my blue car that I built in 1979. I never noticed any piston noise. Now I'm wondering if I still have the TRW instructions filed away.
Even thr older hone machine would measure the amount of amps drawn as it stroked and you could dwell in a certain area to help take taper out
Most modern boring and honing machines that can be set for more dwell time near the bottom of the Bore can both get a cylinder round again if you have 002"to play with and remove most of a taper that a Bore might have. The taper is the tuff part though. To get the taper nice and close you may need to live with .0005"more piston to Bore clearance then you where hopping for. If your going for all of this type of extra work then some how get a deck plate to bolt on along with the head gaskets and hardware you will use.
The issue your going to run into is that the piston/wall clearance is built into the piston.. So your STD size TRW's were designed for a Standard size bore. Now that particular piston will vary a bit, and you may have a thou or so to play with, but if you take .003 out of the bore to straighten it, your now going to have that PLUS the .003-.004 that was designed into that piston. More simply put, my 4.350 bore 455 Stroker pistons actually measure 4.3455. This allows for the .0045 piston to wall clearance they are designed to run at, in an NA build. JW
Thanks guys. What surprised me is that the forged TRW's measure less .0013" under the bore size. Bore= 4.1875, piston = 4.1862-4.1870. So 0.0005- 0.0013" clearance built into the piston. There were some comments on Speed Talk saying Chevy used forged TRW's in some of their stock engines, and were spec'd for 0.0015" clearance. Ultimately it will be up to the machine shop and their capabilities and recommendations. The worst wear is at the top of the cylinder, up to 0.005". If they can just 'kiss' that and hone the rest of the cyl to .005-.006", I may be able to use these pistons. If not, I'll bore 0.020" over for the cast Zollners. BTW, Pistons are TRW L2078, if anyone has paperwork for these...
I need to dig out my old TRW catalog, but those may not be forged as I would think the part number as is typical for TRW would end in a F.
Yes, there were 2000 series pistons in forged and cast. These look forged. Here's a pic of the underside. They have the long slot under the oil ring which means less skirt expansion than the 'drilled hole' type. And some catalog pics I found elsewhere.
The F stands for what pins. They are forged if they were cast you could see some lines. The 2000 series stands for forged
Do you have the measurement of the old Pistons the width. Plus what's the PIN height on these trw's they might be too far in the hole
Make sure your measuring the piston .500 from the bottom of the skirt.. And speaking of resizing the pistons, I have used these guys a couple of times when we have been freshening up well used race motors. https://www.line2linecoatings.com/ Their slick CC coating works great for not only restoring proper piston to wall clearance, it actually clearances what it actually needs, then stops.. Instead of us just imposing a spec on it. That way you could have them put a honing plate on it, and hone the cylinders perfectly round, and then you hand fit the pistons.. That's your best bet for success on the build with your shelf stock pistons. JW
"And speaking of resizing the pistons, I have used these guys a couple of times when we have been freshening up well used race motors. https://www.line2linecoatings.com/ Their slick CC coating works great for not only restoring proper piston to wall clearance, it actually clearances what it actually needs, then stops.. Instead of us just imposing a spec on it." I used these guys on my numbers matching 401 when a spark plug electrode went through the combustion chamber. Honed that cylinder out to .005 clearance, then sent that piston out ot L2L for coating. Excellent product/service..!
