Modern Vespa

Well I have a couple of questions, is this ok or do I need a wider con rod? Of course I Gunna get wide ass bearing . Also if that's not ok can I grind this down to fit a wider rod or will that weaken the crank? The cylinder kit on this engine is a Athena Racing 70cc, crank is a bzm crank, crankcase is custom made. I can't fit a wider Rod on this crank without grinding that because than the crank will rub the crankcase. If I can't grind it then I guess I'll have to get a custom rod which btw who makes custom rods?

Think about this - when the piston is located in the bore, how far sideways (relative to the crank) can it move?
Also - grab the conrod and see how far sideways (between the crank webs) it can move.

The answer is (in case you were wondering) is bugger all.
I wouldn't worry about it.

If I grab the rod and pull it as far as it can go (not forcing it) it will run against the crank, what would I do about that? Does that mean I have a bad big end bearing?

Is it actually touching the crank though? They normally have a spacer between the crank web and the conrod on both sides.

It has the spacers



And it does rub

Is that one you put together yourself? Obviously it shouldn't be rubbing.

No, I hadn't tooken it apart yet because I need to figure out the con rod situation and now this rubbing

So if it's new crank, send it back!

Not a new 1 unfortunately. So what do you think it could be? Im thinking big end bearing for no particular reason

If it's the big end, there will be movement up and down felt at the conrod. The traditional way to test is put your middle finger through the little end of the conrod, make a fist and let the crank hang. Then hit your hand with the other hand. If there is movement in there, apparently you should hear a ringing sound from the crank.

Ok I'll try that

ok i did that and i do hear a ringing sound. there is alittle bit of up and down so i think it is the bottom end bearing

Well there you go... time for a new one!

I hope Im not intruding but I rebuild a fair amount of crankshafts for single and multicylinder two strokes and four strokes and may be able to offer some insight:
On a factory Oem crankshaft I have never seen one that would rub the webs if an axial load (side to side) was put on the rod. Not to say that it doesnt exist. I have just not seen it.

The best way to know is to measure the gap between the big end and the web. Many are around .02". If that spec is close and you still have over a mm or 2 side to side movement, you probably have radial (up and down) play as well.
Hope this helps

Joe

I have about 0.02mm the big end. It does look like the crank has been rebuild before. I do have like a tiny tiny bit of up and down movement.

I have a crank from a TS1 240 with the con rod that just barely rubs on the webs. There's a reason why that crank now gathers dust on the shelf and is not still in the engine.

Sounds like you need a rebuild.

Who's someone who does good rebuilds? I obviously can't do it myself

saintscycleworks would be the first one I'd ask.

For the crank rebuild? I didn't know he did that for peeps

Yep http://www.saintscycleworks.com/services.html

Awesome. I'll probably be rebuilding my crank when I'm ready

Just in case anyone's wondering I decided to just buy a new crank. For the price of the rod and rebuild it was the same as a new crank.