Where did this term "chunk" come from?

Big Heavy Chunk of Iron.

If you ever laid on your back on a creeper and tried to drop one out, yeah that's a heavy "Chunk".

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Try the pumpkin from a Terex TS14 scraper. We found that if the transmission failed it may have been caused by a broken tooth in the differential. This required pulling the pumkin out under the scraper and usually in a ditch after it had rained. Crawl under with the wrenches required, a scraper and a tube of RTV sealant. You do not want to drop the diff in the mud, so when you get it out you rest it on your thighs while kneeling in the mud and 6" of water. Inspect the gears and if OK scrape the sealing surfaces and apply RTV and lift the pumpkin up into place. Snug up nuts at 10 and 2 locations. Now try to crawl.out and get your legs working again so you can finish up.