Rebuilt Engine Piston Resistance
 

I acquired a stock Model B short block that was rebuilt and put in storage about 15 years ago. At first it was difficult to turn it over but after flushing out the cylinders with WD-40 I can now turn it through full revolutions with about 25-30 pounds showing on the torque wrench.

However, there is much greater resistance as the pistons are moving through the middle of the cylinders, which is the area of maximum side thrust. There is much less resistance when the pistons are at the top or bottom of a stroke. I pulled one piston and found the ring end gaps to be 15-18 thousands, nice cross hatch on cylinder walls, and both rod ends to be in spec.

Is it normal for newly rebuilt engine (rebabbited mains and rods, new pistons, rings, etc) to exhibit greater resistance as the pistons are moving through the middle of a stroke?

P.S. I only have experience with engines that were partially rebuilt, and never with a fully rebuilt unit that hasn't been run.

Re: Rebuilt Engine Piston Resistance
 

The piston velocity is higher mid-stroke than at the top and bottom.

Re: Rebuilt Engine Piston Resistance
 

Also less mechanical advantage mid stroke, spark and gas make some noise!

Re: Rebuilt Engine Piston Resistance
 

ursus: 25-30 ft/lbs is perfect for a well rebuilt Model A short block.

Re: Rebuilt Engine Piston Resistance
 

Yes, it is normal in rebuilt, or older engines because the piston is moving fast in mid-stroke. It is caused by ring drag.

Re: Rebuilt Engine Piston Resistance
 

Sounds good to me.

I would use regular motor oil to lubricate the cylinders, WD-40 is not a very good lubricant. On a freshly rebuilt engine that has not yet run, it will always be harder to turn the crank when the pistons are mid stroke and much less effort when the pistons are at the end of their stroke. Just oil everything really good, put everything back together and get it in the car. You may want to use some assembly lube on the cam lobes.

My opinion,

Chris W.

Re: Rebuilt Engine Piston Resistance
 

In your case resistance is a function of the engine rebuilder and time in storage. I would finish assembling the engine and run it on an engine stand. Initially, I would turn it over without the spark plugs and use break in oil. This would help to free up any corrosion on the cyclinder walls and get oil into the main and connecting rod journals. Then follow up with starting the engine monitoring the temperature. I would initially run for a short interval and not exceed 180 degree water temperature. Then progressively run longer and change speed. Also retorqued the head when cold. After running for a couple of hours drain the oil and install it in your car.

Re: Rebuilt Engine Piston Resistance
 

Depending on the pistons and rings, the engine will not loosen up until you have 1,000 to 2,000 miles on it.

Re: Rebuilt Engine Piston Resistance
 

When you first start it, do NOT just let it just idle. Run it at about 1000-1500 RPM, varying the engine speed every so often. You need to get the engine speed up to ensure the internal components and cylinder walls get plenty of lubrication.

My opinion,

Chris W..