Torque to spin a fresh engine?

[PrimeMvr]

I'm reassembling a 3.0. The cylinders are all on and the crank turns easily. But it got me thinking...is there a spec for the max torque that should be required to spin the crank with the plugs out once fully assembled? Right now it's about 5 ft-lbs.

It would be a good indicator of something lightly binding or out of alignment, but not bad enough to notice by feel when cranking with a wrench.

I suppose there are too many variables involved in a build. I did not re-ring or coat the pistons.

Thanks,
Steve

[Walt Fricke]

That is an interesting question. For my race motors I have been contemplating low tension rings. This should show up as low torque to spin.

But there are so many variables. I use Torco red assembly lube all over the place. It is viscous to the point almost of being sticky, and has to affect turning. When you get the heads on and all the valves set up, the turning effort will rise quite a bit.

For me, the turning feel comes into play when installing the heads and cam carriers - i want to be able to turn the cam (no chains yet) as easily after I tighten everything down as it turned before I torqued all those cam carrier nuts and then the head studs, to be sure everything was snugged down evenly.

Interestingly, the factory manual does not show a tightening sequence. I was pretty surprised to find that, as all the VW manuals have one for those much simpler motors. For some years I just made up something which was sort of cris-crossy. Happily, however, our Pelican host Wayne Dempsey's book does have a well illustrated sequence. I don't know just where he came up with it, but he is a genuine engineer, so I always haul out that part of the book at that stage of assembly.

[chris_seven]

I would expect that it could take 5lbsft to turn a crank without pistons and rings.

4 cylinder Fords usually need that sort of torque with 15ftlbs not being uncommon with pistons and rings fitted.

[PrimeMvr]

After the case was sealed and with the rods centered with rubber bands, I could just barely turn the crank with my fingers. After the P/C went on I needed a wrench, as expected. I used a bar torque wrench, not click, so the 5 ft-lbs is a best-read on the deflection as it starts to turn. Definitely not scientific

[racing97]

Fully assembled 3.8 rsr 12 ftlbs Plugs out on beam torque wrench after the breakaway torque.
Current Cup engines 24 ftlbs

regards

[AlfonsoR]

Excellent timing on this topic. I agree wth Walt: too many variables, I think the deviation will be fairly significant. Probably preferable to measure without plugs since the main thing you want to measure is friction drag. However, we could definitely compile a database?

You have to be careful what you use to neasure torque too. A beam type is a good approach, but I believe you need to use one with an appropriate range...maybe one designed for 25 ft-lbs or so.

I am about to begin my first rebuild, a 3.2. I will measure torque on teardown for P&C assy and for crank alone. Will also measure on assembly too of course.

[jomalkay]

I was just recently talking to Mike @ JB Racing on this same topic, doing my first rebuild and do not have the "feel", thought this would be a good thing. I was mainly interested in verification of proper piston ring installation, as i am a guy who don't sleep if i can not measure / confirm. The variables do put a bit more complexity into the equation, but as with any good DOE, if variables are identified, marked and sorted into controlled / uncontrolled, (uncontrolled for example the assembly lube used) much of the variable complexity can be removed. Maybe a good place to start is at a lower level;
-crank only
-crank / P&C's only
so on and so forth. Unfortunately, i am just finishing up the long block so i have no actual data to share.
Thoughts & opinions?

[JohnJL]

Wrt rings, there is a good article in Race Engine Builder magazine this month.

[AlfonsoR]

I have yet to start my engine build and probably wont until after July. But i might do teardown in a couple of months and can post that data. I am planning on using a snap-on torqometer, probably the 25 ft-lb model.

[Walt Fricke]

I can't imagine putting a case together with just the crank in it, so I could measure the torque it took to rotate it. Much better practice to put the rods on the crank with it on your bench. That way you can measure the stretch. With just one piece being rotated you would have a large effect from the lubricant you used. If you were a shop, and always used the same lubricant, you might learn something. But if things are checked to be true, everything measured within tolerance, and properly assembled, I don't see what of importance is learned by doing this.

I see these things as more of a go/no go test. If the crank, with just rods attached, doesn't rotate freely, something is wrong. If it hardly rotates at all, most likely you didn't get the #8 bearing correctly installed, and its locating pin is pressing its side and distorting the bearing's bore. Stuff like that.

I once had a cam which was hard to rotate, and I don't think it was because of uneven head bolt torque. I didn't notice this until I was timing it, so it was combined with other resistances. I went ahead with the assembly, and the first time I tried to start the motor the cam seized, and the locating pin sheared the teeth off of the inner toothed vernier piece. Didn't bend any valves, though. Had a devil of a time getting the cam out, as it appears the front journal was where the excess friction was. I knew I had lubricated it, because there was still red lubricant on the inside of the end cap. A shop cleaned up the journals of the cam and cam carrier, and all worked fine thereafter. Neither of us could figure out what caused this - no large grains of sand found, etc.

Well, it sort of worked fine thereafter. After some track events that cam broke. No doubt the seizing had overstressed it. Moral of story: if things seem like they are too tight, don't just go ahead.

If you know how to adjust ring tension (which I'd like to know), you can do what James Bricken does - measure ring drag with the piston in a cylinder and a fish scale.

[faverymi]

Dealing with the same issue. I had the cam towers assembled. It just kept getting tighter and tighter.

I had to put unreasonable force to turn it. I re torque through bolts. Nothing. I under torque them to 15Nm . It felt proper ( I didn't measure )

Took everything apart. Separated the sealed case.(ohhh that hurts) Back to the drawing board. I'm glad this is my hobby and I don't get pay to do this.

I guess would've hurt even more after my first engine start.

Better safe than sorry, You live and you learn!

[racing97]

It is a good Idea when assembling the cylinder heads that you place the cam towers with out sealant over the heads just using the center 8 mm nuts under in the center of the housing under where the cam would be and torque them pull the cam tower back off then using a straight edge see if the heads are level, this is a big culprit in breaking and seizing cams. then apply your sealant and close it up.

regards