Piston change
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Stage III
Joined: Mar 2010
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Piston change
Have an "83 fxrs that I pulled apart 2 change wrist pins and bushings. Original 80" shovel comp ratio is 7.4:1. What work would be necessary to put some +.010 8.5:1 KB pistons in. Cylinder resizing aside, I don't imagine its a plug and play deal here. Head work? Concerned about piston to valve clearances. Any info would be helpful. 
Club Member
Joined: Dec 2009
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From: Wisconsin
There should be no problem as long as you do not run a high lift camshaft and then the higher compression pistons are notched out for valve clearance so you will never really have a problem with clearance !
Intermediate
Joined: Mar 2008
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As stated there should be NO clearance issues,,,I would weight the new pistons to compare to old ones..If the weight is too much + or - you should rebalance the bottom end..I can't say for sure,,but I believe the KB pistons will be heavier ,,,so filing some material will be needed,,Also check pist on to flywheel clearance,,its stated in the istructions but many guys forget to check..
Dave
Dave
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Joined: Jul 2007
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KB pistons compression are based on 135cc heads. Later heads after '81 have 157cc heads. Use the 9.5:1 pistons to get close to 8.5:1.
KB pistons are lighter than stock pistons and the ring pack is lower than stock and the oil ring may wipe over the oil return hole in the cylinders. If so you will have to add crankcase drain back holes.
I have 9.5:1 + .030 in my 82 shovel and I did the case drain back holes.
I run the V-Thunder SHV 4000 cam and have plenty of clearance.
Be sure to follow the top ring gaping to the letter. KBs retain the heat in the combustion chamber and a little extra ring gap is necessary.
KB pistons are lighter than stock pistons and the ring pack is lower than stock and the oil ring may wipe over the oil return hole in the cylinders. If so you will have to add crankcase drain back holes.
I have 9.5:1 + .030 in my 82 shovel and I did the case drain back holes.
I run the V-Thunder SHV 4000 cam and have plenty of clearance.
Be sure to follow the top ring gaping to the letter. KBs retain the heat in the combustion chamber and a little extra ring gap is necessary.
Last edited by ajayrk; Mar 28, 2010 at 09:20 PM.
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Stage III
Joined: Mar 2010
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Hey all! Thanks for all the info and tips. Looks as if it will be a little easier than I was figuring. Still have more research to do..... may pick your brains a little more before I'm done. Thanks again for the help.
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tree88
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