Fitting the pistons on my WWII GMC 270 engine and was wondering what piston clearance you recommend. The specs I have from the military manuals show a clearance of .0037" (bottom of skirt) That's loose by todays standards but does that sound reasonable for stovebolt engines?
Chevy 235, 261, and 265 V8 engine piston skirt clearance from a 1955 2nd Series Shop manual is specified as 0.0005-0.0011 inches. Quite a difference from 0.0037. I would think that GMC 270's would be similar to the engines mentioned from '55.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
I I built my first GMC engine almost 70 years ago. I give them the clearance they called for back when they were built. Those engines were built with almost 90 year old engineering. And they allowed for metal to expand. Back in the day, pistons or valve stems didn't seize when they got hot because they were given clearance. I have known a lot of people wanting to build a quieter engine. They are just not going to be as quiet as a modern engine. We used to say, "they will be plenty quiet after they seize". Here is a manual published in 1958. The top line is the skirt to bore.
Interesting to see the 1958 piston to bore clearance data from crenwelge above.
I did a search through a post WWII 1947 GMC Truck Shop Manual and a 1955 GMC Truck Manual. Below are pictures from each of those showing a slightly different piston skirt to cylinder wall clearance.
Seems like they tightened up the clearance a bit after WWII and then loosened that up just a tad after these years.
I guess a question I would ask is are the replacement pistons WWII period age for your 270 or are they later replacements that might have a little less expansion due to changes to the metals used? Later pistons that may not expand as much may allow for closer tolerances. You may have to test the expansion theory on the new pistons to confirm it is accurate or not.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
Here's the 1943 "Fits, Tolerances, and Wear Limits" publication I've been using for my270. It lists the clearance at bottom of piston skirt to be .00375"
Could be they updated the size by the time of the 1950s publication or there may be a difference between the dome top WWII pistons and the flat top later pistons. At any rate, the difference is only .0005" so I don't think I'm going to worry about it.
Truck engines in the 1940s and 50s were designed to work for a living. That usually involved running wide open all day with a maximum gross weight load- - - -or more likely a big overload. The Cowboy Cadillacs we're building these days might- - - - -occasionally, be tasked with hauling an iced-down Yeti cooler to the local "Show and Shine" tailgate party. Common sense is getting to be a really uncommon thing these days! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
