Piston to cylinder clearance

Duke · Post by **Duke** » Thu Feb 28, 2013 9:29 am

My SB50 cylinder is at the machine shop and I need to know what the clearance should be. I bought the RIK 44mm kit from Taz. Anyone that has done this before please chime in, the sooner I know the sooner its done!

Duke · Post by **Duke** » Thu Feb 28, 2013 7:42 pm

Bueller, Bueller... Seriously , no one has any input on this one?

paulpauly7 · Post by **paulpauly7** » Thu Feb 28, 2013 9:03 pm

tye clearence on o stock cylinder shoul be the same for the over size piston so check the manual and go with that

patthesoundguy · Post by **patthesoundguy** » Fri Mar 01, 2013 1:05 am

paulpauly7 wrote:tye clearence on o stock cylinder shoul be the same for the over size piston so check the manual and go with that

Yup the manual should totally have the clearance in there. Its a pretty crucial number t have before you bore the cylinder. Too tight and the piston won't have enough room to expand when it gets up to operating temps.

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Duke · Post by **Duke** » Fri Mar 01, 2013 7:50 am

The manual specifies .004 as a service limit but does not mention what the initial clearance should be...what do ya'll think?

bakaracer · Post by **bakaracer** » Sat Mar 02, 2013 2:10 am

Sb50 .0014-.0020 is what should be the clearance.

Duke · Post by **Duke** » Sat Mar 02, 2013 5:07 am

Thanks, Baka!

Duke · Post by **Duke** » Sat Mar 16, 2013 12:29 pm

Ok so my Elite ended up with a .003 piston-cylinder clearance and it heat seized at 30 miles in the break-in period. I pulled it apart and found marks on the piston either side of the exhaust port and some scratching on the cylinder walls. I gave it a quick hone and lightly sanded out the piston scuffs . I rode it around yesterday and it seemed to be fine but today it won't start.

Both of my scooters with larger bores are becoming nightmares! Wish I had left them stock...

tazland001 · Post by **tazland001** » Sat Mar 16, 2013 12:39 pm

What is your compression?

Taz

Duke · Post by **Duke** » Sat Mar 16, 2013 1:13 pm

I have 120 psi...

tazland001 · Post by **tazland001** » Sat Mar 16, 2013 4:10 pm

120 is fine. You might have fouled your plug.

Also did you rejet the carb on the 44mm?

Taz

Duke · Post by **Duke** » Sat Mar 16, 2013 4:47 pm

Yeah I put an 82 main in there, plug looked ok. After the seize I got it running but it just sounds awful, kind of rattly and has a big vibration off throttle. I replaced the crank bearings during the rebuild including the small end conrod one. I believe its fubar

Before the seize it was nice and quiet and smooth-running...

tazland001 · Post by **tazland001** » Sat Mar 16, 2013 5:28 pm

The 82 sounds a bit lean. I will chime in on this thread a little later tonight again.

Taz

Duke · Post by **Duke** » Sat Mar 16, 2013 5:38 pm

I think I messed up because I didn't have the clearance figured out before i went to the machine shop. He said the final clearance came in at .003 which is close to the max tolerance (.004) I think instead of a heat seize the piston was slapping and got stuck...

bakaracer · Post by **bakaracer** » Sun Mar 17, 2013 10:42 am

usually to tight a clearance would cause heat siezes easier. piston slap from to much clearance just cause faster piston wear and blow by easier. sounds more like you had either a lube issue from not enough oil and fuel or motor wasnt heat cycled enough and broken in properly or you have a leak somewhere.

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Piston to cylinder clearance

Piston to cylinder clearance

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