Modern Vespa

Although the engine turned over freely and evenly as I was assembling it, it is really hard to turn and uneven once I put the flywheel side case on. It seems to be free in the phase when the piston is all the way out, but really tightens up as the crank comes around and sweeps across the rotary pad. With the cylinder and head on I can hardly turn it at all.

Is the crank possibly not all the way centred, or is there something else I should be looking at?
I tried to attach a video so you can see the effect but I think the file is too large so we will have to make do with this series of three screenshots. All and any suggestions much appreciated.

You have to upload video to a service such as YouTube and link it here.

I would pull the clutch and check again, If no luck, pull the fly side case. Work backwards until you find the source. Is the engine in neutral?

You're on the right track thinking it's crank alignment issues.

I usually leave the crank puller on until I get the center case bolts on.... turning the crank and checking during the entire process.

Good luck

That's a good strategy. When working on other engines, my method when mating case halves has been to start with the fasteners closest to the crank, tighten gradually while checking rotation, wrench tight, bring up torque values 5 lbs at a time, check rotation as I go. If any time during that process, something binds or does not feel right, stop and find out what it is.

In one of the SIP engine building videos, they do a dry build kind of assembly where they mate the cases and check things individually; mostly to ensure the x-mas tree and gear set line up and the cruciform is correctly lined up in each gear. This is kind of what I have done with automotive engines; cam in case by itself, crank in case by itself, crank and cam together. Kind of overkill on a Vespa because fewer moving parts but the general idea applies.

Sorry to blather. Good luck.

Thanks for these helpful thoughts. I have removed the cylinder and head, taken off the clutch and the flywheel side of the case. The crank spins freely. I am looking at whether it might be slightly out of alignment - enough that when I put on the other half of the cases and torque the bolts it binds on the flywheel side.

Looking at the gaps though there doesn't seem room for the crank to come further towards the clutch side. See pics. There is a tiny reveal at the bevel on the bearing where it meets the shaft on the crank. Should it align with the base of the bevel or flush with the top edge?

The bearing should be flush with those two brackets. It looks correct in your photo but it's hard so tell. The bearing in my Stella case looks similar. One side is machined to a different depth. Is there a snap ring on the other side of the bearing.

What method did you use to install the crank?

That was my next thought…

The crank was installed using a puller.

Get another crank.

That one's shot.

Your issue:

Thanks so much for your help. Thank Ray8 for identifying the problem. I'm pretty ignorant- can you break it down for me: what is wrong and how do I fix it?

..and when I say that I mean how do I ensure the new one doesn't replicate the problem

Wow, good eye! I'm not quite sure what I'm looking at on the crank. Looks either mangled or covered with JB weld on the fly side. Can you post a better pic, just to satisfy my curiosity.

Always nice to find a smoking gun even when it costs money. Better than weeks of chasing a mystery.

Yes, it is really good to finally identify the cause of the problem. I have had this apart and together multiple times. Thanks everyone for your help. What is on the shaft is grease, smeared there by me to facilitate mating the flywheel side case. Here are some more pictures of it that part of the crank.

Hmmm…I'm not seeing anything, but I'm quite inexperienced with Vespa engines; bent crank? Scored bearing taper? I'm not sure what was seen. I'm sure someone will be along to point it out.

Looks like crank is still not all the way in.Get the pullar out.Either bearing may also be sitting on a very slight angle (not seated so give them some encouragement down flat with proper tool or big socket) Good luck
After that go shopping for a crank

If the crank is twisted (happens often) replace it with a 60 crank and 1mm base gasket. Similar money but with free speed.

Thanks for all this helpful advice. I am proceeding by pulling the crank to see if i can find somewhere that the bearing is not properly or totally seated. If I can maybe it means I have don't have to replace the crank…

These pictures of the bearing from the clutch side seem to indicate it is flush with the two tabs and square on. The pic from the interior shows two interesting things to note though: the oil seal is a tiny bit proud with a slight lip (i can just catch my nail on it) from about 1 to 9 o'clock. The other 1/3 of it is all the way flush with the case surface. I have tried hard but can't get it to you bed in any further. Even more tellingly, there is a clear pattern of wear in the case surface where the same section is rubbed smooth. Clearly something is offset. The question I have is does this look like the issue and if so how to fix it? Does this tell us the crank is bent (this wear pattern predates my rebuild)? Or can I cheat the bearing inwards a thou and tighten the clutch side tab that lines up with this wear pattern down the same amount and "correctly" align it?

Have you had the engine completely back together and fully torqued?

I had some alignment issues initially on a 50 special engine. Initially the crank was binding and then after I fixed that the driveshaft, both fixed themselves as I got all the case parts back on fully torqued them now it's all spinning freely.

I. used one of the crank puller tools to fully seat the clutch side it also helped to keep the aligned too.

Thanks for the suggestion. Unfortunately yes I had it back together and fully torqued twice. That's when the symptoms are most pronounced.

The wear pattern on the cases are nothing to worry about… sometimes new cases have that.

Seems to be part of the machining process

The tolerances on the cases are usually pretty forgiving, even if they're not straight, they don't bind to the point things don't spin.

I'd imagine the culprit will be your crank, driveshaft or primary…

High chance it's the crank, especially if it's old

Hold up. Does it bind at the same rate 360 degrees? Swap the crank for an easy fix.

The seal being slightly proud shouldn't be a problem, I don't think. The crank can only go into the bearing so far before it hits the shoulder on the crank.

You could take the crank to a general machine shop and have them measure it with a dial indicator mounted on centers. A motorcycle shop could probably measure it and straighten it.

I would have it checked for runout and replace it if it isn't straight. If it turns out to be ok, keep looking.

Based on all the helpful input here I inspected the crank to try and determine whether to replace it, take it to a shop to be trued, or persevere. On the basis that it looked measured pretty straight (at least with the limited tools at my disposal), and had been turning over okay when I took it out, I thought I would try reseating it one more time.

This time I started pulling it on using the clutch technique before switching to the crank puller. It seemed to seat more easily, and after torquing up the case bolts it finally now spins pretty freely. Hurrah!

It isn't entirely smooth all the way through the cycle - it gets slightly harder to turn in one spot which I think is as it starts to cross the rotary pad - but it is way improved. Does that seem normal - forgive me but I don't know what normal is in this situation.

You shouldn't feel any resistance at any point in the rotation and the web certainly should not make contact with the pad.
It's kind of hard to tell if it's smooth all the way around with the con rod doing what it does, but there should be no binding.

If the crank is encountering resistance as it crosses the rotary pad, something is off. It shouldn't scuff the rotary pad and don't keep doing that. Because that is $$$ to repair.

Have the crank measured for realz. Any machinist can do it. How much is your time worth? Can you place a dollar value on the time you have already spent opening and closing the case? Now factor in the frustration and ass ache of doing that. Has it cost as much as having a machinist measure it…or even a new crank yet?

How did the rotary pad look? Anybody had their peanut buttery fingers in there with the JB weld?

Props for keeping at it and chasing down the problem, but sometimes you have to get help to move forward.

Thanks again everyone. Today I took the cases apart again and checked the clearance on the rotary pad. All good - no issues with peanut butter! This time zipping it up it was smooth with no bumps until I got the cylinder on and then it really tightened up. I took it off again and it turned over as free as could be. So flipped the whole thing on its end so I could put the cylinder and head on without gravity getting in the way. I cinched up the head bolts and bam! it's turning really good now. Not sure I can entirely explain it, I'm just grateful it is working now.