Bob Bauer 26 #1 Posted June 30, 2023 Hi , so I’m rebuilding a k 91 off a rj 58 and want to order piston and rings , there are three different options one is standard,the other one is .010 and a .030 over . So which one would be closer to this engine because I have no clue or does it matter because it will have to be bored out anyway .thanks Share this post Link to post Share on other sites
Jon Paulsen 290 #2 Posted June 30, 2023 The size depends on how far they have to bore it out to get a good, clean surface. These numbers are 1/1000ths of inches. So your options are no boring, bore 10/1000" oversize or bore 30/1000" over. You might mention to the shop who will do the work that you can only get standard, .010 and .030, with no .020 available. So, they will need to just hone it a little, or go .010 over size or .030 over. If they go .020, you will have to find .020 rings for it to work. If it's not severely damaged, they might be able to just hone it (you could do that at home), or maybe take off .010. If it's in bad shape, they might go .030. It depends how deep the damage is. 4 Share this post Link to post Share on other sites
WHX?? 48,821 #3 Posted June 30, 2023 Hello Bob... best to let the machine shop do the measuring and telling you how far you need to go. Unless you have the instruments to do it your self. Be sure to check taper and out of round. With whatever size you end up with the shop should fit the bore to the new piston since the pistons available nowadays can vary. 3 3 Share this post Link to post Share on other sites
Bob Bauer 26 #4 Posted June 30, 2023 19 minutes ago, Jon Paulsen said: The size depends on how far they have to bore it out to get a good, clean surface. These numbers are 1/1000ths of inches. So your options are no boring, bore 10/1000" oversize or bore 30/1000" over. You might mention to the shop who will do the work that you can only get standard, .010 and .030, with no .020 available. So, they will need to just hone it a little, or go .010 over size or .030 over. If they go .020, you will have to find .020 rings for it to work. If it's not severely damaged, they might be able to just hone it (you could do that at home), or maybe take off .010. If it's in bad shape, they might go .030. It depends how deep the damage is. Thanks a lot 1 1 Share this post Link to post Share on other sites
Bob Bauer 26 #5 Posted June 30, 2023 5 minutes ago, WHX?? said: Hello Bob... best to let the machine shop do the measuring and telling you how far you need to go. Unless you have the instruments to do it your self. Be sure to check taper and out of round. With whatever size you end up with the shop should fit the bore to the new piston since the pistons available nowadays can vary. Thank you Share this post Link to post Share on other sites
ri702bill 8,327 #6 Posted June 30, 2023 And, do not forget the piston's partner, the Connecting Rod. The crankshaft journal MAY need to have a service where it is reduced by .010 if it is severely worn AND the journal has NOT already been turned smaller.... needs to be accurately measured. The top of a Kohler pistion is usually stamped "STD" from the factory, signafying a standard bore... It MAY be stamped ".010" or ".030" if it has already been machined. The .030 overbore is the accepted limit....but the block MAY (and I do mean may) be able to to opened up further to accept a pressed in sleeve to take it back to STD size..... but from then on, its ability to do heavy work has been compromised. 3 Share this post Link to post Share on other sites
ri702bill 8,327 #7 Posted June 30, 2023 (edited) 1 hour ago, Bob Bauer said: With whatever size you end up with the shop should fit the bore to the new piston since the pistons available nowadays can vary. So true for both parts. The Machine Shop NEEDS these new service parts before final sizing. They need to try to size the final clearances for "Best Fit" within the allowed tolerance to assure long engine life.... Edited June 30, 2023 by ri702bill 1 1 Share this post Link to post Share on other sites
Bob Bauer 26 #8 Posted July 1, 2023 15 hours ago, ri702bill said: And, do not forget the piston's partner, the Connecting Rod. The crankshaft journal MAY need to have a service where it is reduced by .010 if it is severely worn AND the journal has NOT already been turned smaller.... needs to be accurately measured. The top of a Kohler pistion is usually stamped "STD" from the factory, signafying a standard bore... It MAY be stamped ".010" or ".030" if it has already been machined. The .030 overbore is the accepted limit....but the block MAY (and I do mean may) be able to to opened up further to accept a pressed in sleeve to take it back to STD size..... but from then on, its ability to do heavy work has been compromised. Thanks Share this post Link to post Share on other sites
Bob Bauer 26 #9 Posted July 1, 2023 15 hours ago, ri702bill said: So true for both parts. The Machine Shop NEEDS these new service parts before final sizing. They need to try to size the final clearances for "Best Fit" within the allowed tolerance to assure long engine life.... Thanks Share this post Link to post Share on other sites
