The Stovebolt Site Forums The Shop Area Engine Shop Boring a 270

I got the block stripped down on my '56 and have a couple issues I cant find answers for by searching. Nothin I saw anyway,...

So, I open her up & the pistons are all stamped with 30, so I'm assuming it's been bored at least 30 thou over already. On top to that there was a generous ridge that I had to ream out before droppin the pistons out, I'd say it was easily more than 10 thou. Even with the wear (pretty uniform on all 6), the #2 has a nice chunk of pitting at the top from when the engine sat for a while & rusted.

Here's my questions:
How far over can you bore these ol pigs? It's not gonna be a hot rod.
The 270 is a 4" bore, could I bore it out to 4.25" & use 292 pistons (assuming they have the same pin height that is)?
And more importantly, anyone have a couple links for suppliers that carry these pistons & rings?

Thanks for the help!

The 302 has a 4 inch bore. You could overbore it up to about 1/8 inches or .120". For practical purposes it would not be wise to go more than .060".

You should mic the cylinders to see exactly where they are at, and go with the smallest overbore necessary to get them all clean and matched.

4.25" not a chance! 4.030"... maybe

A lot of nos pistons come up for the military 302s. Maybe one of those will work for your 270. The pin height is the same for 270 and 302 gmc motors.

Thats good to know, I dont find much of anything for the 270, but parts for the the 302 are all over the place.
*edit*
Just looked up the casting numbers off my rods & found they are the 302 rods that have a 1.050" wrist pin vs. the .990" pin in the 270.
Neat casting listing I found on old GMC trucks website. All of the parts on the bottom end are stock other than the rods.

As for the overbore,... Pulled it all apart today & got a good look at all the jugs today. #2 is still the worst, but 3 & 4 are pitted pretty badly as well. I'm hopeful I can get away with .60, but not holdin my breath on that one. Gonna take my bore guages over tomorrow to get some prelim numbers & see if the shop has a depth guage I can measure the pitting with.

The GMC blocks can be sleeved by a machine shop to take care of the pitted cylinders. You may want to look for another block, though, GMC's are expensive to rebuild if you have to source new pistons.

4 on the floor;
I've noticed that! Any lines on low priced pistons for a 270? Mine aren't beat up too bad except they rattle like a diesel every time I back off the throttle under compression. Doesn't act too life threatening,but also doesn't inspire confidence to do road trips etc.
Speed

I've been doing some measuring & came up with some numbers that aren't good, but aren't all bad either.
The limitation on the 270 with a .040 overbore is 3.820-3.822. My fattest cyclenders are 3.181, but I do think some of the scoring in the rusted ones is still too deep for that.

Maybe there is hope if the 302 pistons have the same pin height, since the standard bore is 3.999-4.001, it's not much of a stretch from the 270 bore. even if the pins heights are different, there's enough 4" pistons out there, I should be able to match something up from the vast array of V-8 parts out there. I hope.
Time to blueprint one of the pistons & start digging.

Freeq2002 -- just sent you an email

I was just reading the GMC speed manual by Bill Fisher. He mentions boring the 270 out to 3 15/16 to come up with a displacement of 292 cubic inches. For the 302 he reccomends a 4 1/8 bore to give a displacement of 320 cubic inches.

I might have to go back on the 302 pistons... the 4 inch bore may be wearing those cylinder walls a little bit thin.

If you are thinking about a heavy over bore, the block should be sonic tested to determine the thickness of the cylnder walls before doing anything. Egge offers the pistons for the 270. My bet is that the ring groves are worn out on your old pistons and the slapping you hear sounds like skirt wear. The std bore on a 270 is 3.781, my guess is this thing won't be able to handle a 4.00 bore, thats a difference of .219!! I would have a Machine shop check the bores and see if it will go .060. Good luck.

Sean

Haveta make sure the stroke & pin height on the 292 is the same as the 270, then I can get pistons for that, saw loads of them on one manufacturers web site, more than the 302s anyway.
My +.030 pistons look real good, they were replaced back when the head was salvaged from the junkyard and other than one cracked ring, they look good.
Ah well, I can still have the block sleeved back to the standard 270 bore, just trying to avoid too much expense, but then again there's the added cost if I blow out a cyclender as well. Lovely stuff!

Lookin like I wont have the powerplant ready in time for the local Cruzin' the Harbor show on Mar 31st. Not enough free $$ to get it all done that fast anyway.

The stroke and pin height on a 292 is definitely different than that of a gmc 270, so the pistons for a 292 will not work without modification. The 292 that I mentioned above is the displacement of a 270 that has been bored to 3 and 15/16, which should also be considered a maximum bore for the 270. My guess is that if it was possible to get 302 cubic inches out of a 270, Bill Fisher or Frank Mcgurk would have done it.

Talk to Patrick Dykes or Tom Langdon. I remember reading about Langdon's 302 that has been punched out to 340 or something like that using more common v-8 pistons and rods. Machine work would have to be done on the crankshaft, cylinder walls, and head for that to work. If anybody would know it would be Tom. It has been said that he has forgotten more about the 270 than most will ever know.

If you are just looking for stock replacement pistons then get military surplus ones. Both the 270 and 302 were in military vehicles and parts are around. However, if you are building it for performance then either Egge or Veniola will have pistons. It won't be cheap.

I'm pretty sure the 340 GMC is a 302 bored out to max and also stroked by offset grinding the crank. Not for the faint of heart or wallet.

I'm keeping her pretty much stock, and while I am not in the fiant of heart category, the military dont pay so much that I cant say I dont fall into the faint of wallet one.
I was looking on a website for one piston company that makes stock re-pops of all sorts of engines. I didn't see any 270's, but there was a load of 292 & 302. I'm gonna get one of the pistons form my engine & do some measuring on it to get exact pin heights, pin thickness, all that fun stuff, and bounce it off them. I'm relatively certain I can find another piston from some other engine that will work fine in the 270 bored over to 3-13/16".
Of course, all this will be in the air till I can get the block in to be magna-fluxed before I bore it, gotta make sure it's a solid block. Gonna do that shortly, just wanted to see what sort of bore I was looking for 1st.