3750 mark II vs 670I
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=204935
Printed Date: 15 Jan 2025 at 9:34am
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: 3750 mark II vs 670I
Posted By: RockingJFarms
Subject: 3750 mark II vs 670I
Date Posted: 13 Jan 2025 at 4:34pm
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I've got an 8070 that got cooked bad and the head, crank, and most likely block are shot. I have found a motor out of a 7580 that would be a good rebuild candidate. Rods, injector, injection pump, turbo, and intercooler are salvageable from the 8070 so those will all get swapped over to make it a 670I. The only thing I can't find much info on is difference in cranks and does it really matter. The parts number for the crank in the mark II is different then in the 670I, same with the dampener. In the grand scheme of things does this really matter? The internet says mark II had a weaker crank but I can't find any verification and this tractor will turn at most 200 on the pto, so it leaves me to wonder if it would really make a difference. Anybody have any insight on this?
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Replies:
Posted By: DrAllis
Date Posted: 13 Jan 2025 at 4:52pm
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Assuming the 3750 engine has an original crankshaft, it is probably different than your 8000 series. If the bolt that retains the crankshaft pulley is a 12 point head that takes a 5/8" or 11/16" socket, that is the newest crankshaft. The 3750 bolt takes an 1 5/16" ?? socket or an 1 1/2" socket. I'm pretty sure the first 8000's had the same size bolt but the crankshafts steel was upgraded to 4140. A couple of years into the 8000's they went with the smaller head bolt. The 4140 crank should be stronger than the 3750 and the latest crank is the best of the three. 3750 pistons are 4-rings with a 1 5/8" ?? piston pin and all 8000's are 3-rings with 1 3/4" piston pin. So rods are married to which piston you use. There's a piston cooling difference too. FWA tractor engines and 4-wheel drive engines never last as long as a 2-wheel drive with duals does at the same HP level. The poor 426 in an FWA or 4-WD is getting the crap worked out of it because the chassis has sooooo much traction. I guarantee your engine will last far longer at 180 PTO HP than 200 HP.
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Posted By: RockingJFarms
Date Posted: 13 Jan 2025 at 5:59pm
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The 3750 would be getting refreshed, so new pistons which would be 8070 pistons, re use the bigger rods from the 8070, new bearings and caps with ARP studs. I would heavily assume the 8070 is a late model as it came with a khd badge on the front. The real question is will the rods fit (I noticed bearing numbers are different too) and will the strength difference between the mark II cam and a 4140 cam make a difference in a tractor making relatively stock power running at 1850-2250 rpm?
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Posted By: DrAllis
Date Posted: 13 Jan 2025 at 6:06pm
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If you are using 8070 conn rods, you must plug the piston cooling jet hole in 6 of the upper main bearing saddles so you don't have double piston cooling, which will hurt your engine oil pressure when hot. I'd never run one of those engines that slow, expecting it to produce a fair amount of HP. Keep it at 2200-2400 RPM for better results. I can't predict crankshaft strength/life. Make sure to have your crank magna-fluxed for any cracks and maybe even remove the crank gear and mag around the half-moon key way for the gear. If it isn't a STD/STD crank, I would never use it in this application. Use STD/STD only.
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Posted By: SilverShoes
Date Posted: 13 Jan 2025 at 6:35pm
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That’s a great conversation to read, thanks to both of you for digging through these things and thoughts
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Posted By: tbran
Date Posted: 14 Jan 2025 at 11:45am
A little trivia - the injector lines on the N6 have a larger ID than the ones on the N5. Same on the 7080/7580 vs the 7060 and down... Bosch and Roosa
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When told "it's not the money,it's the principle", remember, it's always the money..
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