compression

[norcal blaster]

I went out riding yesterday and the bike ran like a champ. Today i washed it then tried to start it and when i went to kick it there was little tension on the kick. I feel and hear grinding when i kick. There is no compression. Could it be rings or the whole piston?

 

[scotj77]

going to have to pull the top end to figure that one out could be lots of things. little tension doesn't sound good. nothing happened while you were ridding? it wasn't acting funny right before you shut it down?

 

[Awk08]

i'd pull the clutch cover and give the kicker spring and gear a look first, seeing as how it ran till shutdown, but a compression test will tell any signs of internal troubles without wasting a top end gasket set

 

[scotj77]

right. I wasn't even thinking kickstarter. def possible

 

[brandoz28]

i'd pull the clutch cover and give the kicker spring and gear a look first, seeing as how it ran till shutdown, but a compression test will tell any signs of internal troubles without wasting a top end gasket set

that's what i'm thinking too, a kickstarter gear may be wasted too. i'd do a compression check with a gauge, see if you get anything at all, then check under the clutch cover

 

[norcal blaster]

I took the top end apart and found the piston was done but cylinder is still in good shape. Also, the crankshaft bearing was done also...that was the grinding sound. My question is the top of the piston is stamped 72.45mm...that means it has a 240 kit? I checked the Vito's performance website and the only big bore piston replacement is 72.50...do i have to re-bore for that size?

 

[brandoz28]

yes, if you have a 72.45 piston and its spent, it might not clean up for a 72.50...which means that jug may need a sleeve. pics of the jug may help to identify it

 

[norcal blaster]

Does Vito's make a 72.45 piston?

 

[brandoz28]

i don't believe so, why is the piston spent exactly? are the cylinder walls still in good shape?

 

[sicivicdude]

Alright, a lesson in vito's version of bore size. The vito's kit "stock" (if you want to call an aftermarket jug stock) size is 72mm. 72.0000000000 mm <--- well maybe not that many zeroes but you get the idea. In order to allow the .002" piston clearance with a bore size of 72.00mm they have the piston machined to 71.95mm which is duh-duh-duh .002" smaller than the bore size.

For a .5mm overbore, the cylinder size is 72.50mm the piston is 72.45mm.

Norcal, you need to take your cylinder to a machine shop and have them check the cylinder for out of roundness and taper as well as out of bore. If the 71.50mm bore is shot vito's can get you a 73mm BUT that's the last one available. So before you start it back up, leak check it and immediately do some plug chop runs after break-in. Don't chance it with "it'll be ok".

 

[scotj77]

can't believe you weren't doing cartwheels when you discovered you had a big bore kit on it already. send that jug off to ken o'cconor and have him take care of it for you. I would send your crank to him too and have him make sure it is within specs even if you buy a new one. good luck with the rebuild. and listen to all the advice you are getting from these other guys too. If you follow them explicitly you will not have any issues with your new top and bottom end. leak down test is a must.

 

[norcal blaster]

brandoz28-the cylinder walls are still in good shape...it is still smooth all the way around. There is no scratch marks or grooves.
Sicivicdude-So if i have a 72.45 piston right now and the cylinder is still good...i can run the 72.50. Right?

 

[royalt67]

Yes and no. He was saying have the cylinder measured to be sure it isn't worn beyond limits. Glass smooth is beautiful but it can still be worn beyond proper clearances. You throw in another piston, it might work fine, but wouldn't it be good to KNOW? Plus you still need to hone it for rings to break in. If it is close to max tolerances, a hone might push it over and then piston slap-slap. Get it checked.

 

[norcal blaster]

Thanks Fellas!!!

 

[Budget_Blaster]

Glass smooth is not good it indicates glazing and glazed cylinders don't allow the rings to seat properly.you need to have a shop check it out no matter what you decide.I've built a lot of engines mostly small block fords and seen perfect looking cylinders be out of spec.in fact I got a 85 gt roller motor with perfect cross hatch even that when I miced the bores was out of round and needs bored and this thing looks like it has no wear not even a ridge built up arround top!

 

[norcal blaster]

Should i even bother trying to get that cylinder checked out or should i just get a new kit? I has already been bored out...

 

[scotj77]

get the cylider mic'ed to see if it is still usable if it is get the 73mm piston and boring done. make sure they chamfer the ports though.

 

[sicivicdude]

Yes you should bother gettting the cylinder bored. It's MUCH cheaper to get a new piston and a bore than a whole new cylinder.

Once the cylinder done and no more bores can be performed THEN you contemplate getting a new cylinder or installing a sleeve.

 

[norcal blaster]

The top end is at the shop...Will post something soon as i get it back.