3750 mark II vs 670I

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.

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.

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

Edited by tbran - 18 hours 20 minutes ago at 11:46am