250 Crank Work

[Rick]

Stan,
It's definitely a problem knowing where to press/wedge/hit- I draw pictures and think I know where to hit and half the time it goes the wrong way. You have much more experience at this than I do, so don't abandon a method that has worked to take my advice.
Using the crankcases just seemed clumsy to me- pushing and pulling the crank through the bearings several times(you probably get it right with fewer attempts), and, if it was way out of alignment it probably wouldn't even fit through both bearings. It just seemed like using the crankcases would let you force the crank to distort enough to fit and show acceptable runout when the alignment wasn't really that good, or there was lots of stress built in that would relieve itself someday.
Truing a crankshaft with a hammer is just too unpredictable to satisfy me, so I'm building an accurate press. I saw a photo of the factory press one time, and it looked just like the 4 post die sets that I used for years. The top and bottom plates are line bored to fit the crankshaft journals, and the top plate slides on the posts through low friction linear bearings- it should be possible to press a crankshaft together as accurately as you can machine the bores in the plates- or so I hope.
Rick

[Nick]

Not only do engines put out more power when they're hot, they put out more power when they're allowed to 'grow' and to 'flex'. That is, if the engine has a top motor mount of some sort, it should be one which allows the engine to expand vertically as it heats up. Fortunately our Ducs have no top motor mounts.

Re: truing cranks in the cases, I wouldn't install a crank like that until I'd confirmed the readings on a stand. You wouldn't want to get the bike together only to discover that it had a nasty vibration.

Whoever balanced my Sebring's crank at the factory did a marvelous job, because the bike is dead smooth at highway speeds.

[Jordan]

There are only 4 points to hit the flywheels if they're out of alignment, all at 90 degrees from the big end.
I hold the assembly by the con rod, let it dangle at knee height, hit it with a lead hammer.

I''ve seen Ducati professionals slam it down onto a steel slab on the floor, but that made dents in the flywheel rims - but it was fast!

[Stan Lipert]

My crank truing method:
The goal is to press with the crank webs parallel, checking on the press before applying pressure, with a straight edge. If in doubt, take out when half pressed, check and true before pressing all the way to the pin shoulders:


The webs are checked to be parallel with a straight edge (digital caliper) around the crank circumference:


There is always one web that is larger than the other, so the smaller web needs the gap equal around the crank.

I only hit to true at two points: I hold the flywheel side by hand, and hit the high web at 90 degrees:


My hammer is a 1.5 pound brass, hit quite hard! No dents left in the steel web.

After the gaps are equal, I measure the thickness at the pin, and compare around the crank:


If the crank is wide opposite the pin, I lay the crank on a table edge, and give a moderate hit with the deadblow hammer:


If the crank is narrow opposite the pin, I use a cold chisel to spread the crank:


Repeat the measuring and truing as necessary, before checking in the cases for runout. The first check, the runout should be within 0.004". I'm happy with a total runout of 0.001"

Rick,
I've rebuild about six Ducati singles, and many modern twins. All of the cranks drop in without force, unlike many Japanese vertical split engine that have an interference fit of the crank journal to the inner bearing race. There is no built up stress when the crank is installed in the cases. The cranks should be able to move left-right at room temperature.

Frank Giannini has built his own crank press jig, If you don't want to do your own, you can have him press the crank- cell 973-207-9086.
I don't do enough to bother making a jig.

[Nick]

Thanks again for all the pics & info Stan.
When I was truing Husky and CZ cranks regularly back in the day, we rested one wheel on the wooden bench before hitting the other wheel with a lead hammer. However, holding the crank, as you did, is probably better.

(Funny, they used to talk about high quality 'Swedish Steel'. What a joke that was! Compared to the superb quality of the CZ's crank and gears the Husky bits seemed to be made of pot metal. For every CZ crank we rebuilt we did 10 or 15 Husky cranks.)

Regarding your comment about the crank turning freely in the cases, in his book 'Tuning for Speed', Phil Irving writes that to check whether everything is spinning as freely as it should, one should assemble cases and crank, then install piston on rod without rings and install cylinder. Then, when giving the piston a sharp push downwards, the whole plot should spin with absolute freedom and the piston should run in the center of the bore.

The book is fantastic and belongs in every engine builder's library.

[Stan Lipert]

Nick,
Yes, I have assembled my engine bottom end, trans, clutch complete, piston but no rings. In neutral I can push down the piston and it will have enough momentum to spin the crank and clutch to come back up on it's own. I have a .pdf of Tuning for Speed, but need to read it some more. I like your method of resting the crank on a wooden bench for hitting. That may work better than holding it by hand!

[DBDBrian]

This is a jig I made to press the crank apart.I tend to think there is less chance of damage if things are held in line.

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P1040629.JPG

When truing a crank after assembly, I use a slug of lead about the size of a large orange, holding the crank in my lap, I know it sounds a bit brutal, but the lead don't mark the wheels, and a shock action is what is required for fine adjustment, as said a press creates a jump action.
I check alignment with the crank running on bearings in V blocks on a surface plate. With a dial gauges running on each end of the shafts, this gives a good indication as to what is going on, and the action required.
A jig to insure good alignment when pressing up the crank can not be a bad thing.

I have a grinding wheel balancing jig, but this has knife edges, so is no good for alignment. but is ideal it for balancing cranks.
I have attached a picture of the jig as I thought it might be of interest, apologies for the crank not being a Ducati, it's one I made for my Goldstar.
Brian