Mighty Shee Posted December 27, 2019 Report Share Posted December 27, 2019 Had piston slap after running motor for about 30 min. with no left K&N filter pod at the dunes. My cylinders compression was at 163 (left) 165 (right) after being ran with no filter and before I took apart the top end. I removed top end and took cylinders to CT Racing In Southern California. He honed my cylinders after he received my new Pro-X piston to match them to the hone. I put my top end together with the new pistons and performed a leak down test (she held 7 psi for 7 min). When I did a compression test using the same tester I used before I put together my honed cylinders with new pistons it’s now reading 118 (left) 120 (Right). Is it possible that the new hone would have caused the compression to go down? Sent from my iPhone using Tapatalk 1 Quote Link to comment Share on other sites More sharing options...
the boris Posted December 27, 2019 Report Share Posted December 27, 2019 Had piston slap after running motor for about 30 min. with no left K&N filter pod at the dunes. My cylinders compression was at 163 (left) 165 (right) after being ran with no filter and before I took apart the top end. I removed top end and took cylinders to CT Racing In Southern California. He honed my cylinders after he received my new Pro-X piston to match them to the hone. I put my top end together with the new pistons and performed a leak down test (she held 7 psi for 7 min). When I did a compression test using the same tester I used before I put together my honed cylinders with new pistons it’s now reading 118 (left) 120 (Right). Is it possible that the new hone would have caused the compression to go down? Sent from my iPhone using TapatalkI'm not a mechanic but I'll give you my opinion.I don't think hone would drop compression that much,especially if you changed pistons to match as well. What gasket thickness were you running before the refresher and what are you running now? Sent from my VOG-L29 using Tapatalk Quote Link to comment Share on other sites More sharing options...
Mighty Shee Posted December 27, 2019 Author Report Share Posted December 27, 2019 I'm not a mechanic but I'll give you my opinion.I don't think hone would drop compression that much,especially if you changed pistons to match as well. What gasket thickness were you running before the refresher and what are you running now? Sent from my VOG-L29 using TapatalkThe Cometic gasket kit I was using before the hone is a 65.5mm kit (C7094). The new kit sold to me by the shop that did the hone is a 68mm kit (C7316). I just called Cometic and the Tech. there told me that both those kits should be the same thickness. I don’t have calipers to measure my exact bore, so could say exactly what my bore is. Maybe I’m losing compression cause my gasket kit is too big for my bore. Sent from my iPhone using Tapatalk 1 Quote Link to comment Share on other sites More sharing options...
the boris Posted December 27, 2019 Report Share Posted December 27, 2019 The Cometic gasket kit I was using before the hone is a 65.5mm kit (C7094). The new kit sold to me by the shop that did the hone is a 68mm kit (C7316). I just called Cometic and the Tech. there told me that both those kits should be the same thickness. I don’t have calipers to measure my exact bore, so could say exactly what my bore is. Maybe I’m losing compression cause my gasket kit is too big for my bore. Sent from my iPhone using TapatalkThat's quite possible, your base gasket plays big role too. It would be good if you can check sqush too. Sent from my VOG-L29 using Tapatalk Quote Link to comment Share on other sites More sharing options...
bansheesandrider Posted December 27, 2019 Report Share Posted December 27, 2019 Have you fired this up and seated the rings yet? Did you lube the piston and rings when you were putting it together? Did it have a factory style head gasket when you took it apart? What were the base gaskets-stock thickness or something different? I have never been fond of the Cometic stuff, if I am building a stock type motor with just a cleanup bore/hone I prefer to run genuine Yamaha gaskets. Is there more to this build you are not telling us-high compression head or big bore or stroker or did it have some other specialty piston that raise compression? Was it carboned up when you took it apart- if it had enough piston slap to be noticeable, I wouldn't think it would make the kind of compression that it had if everything was normal. Did they hone the cylinders to the new pistons or was it BORED to the new pistons? What is the actual bore size now? Unless they bored it way out you shouldn't need the 68mm gasket set, you should have stayed with the same as what you had before or OEM gaskets. Quote Link to comment Share on other sites More sharing options...
Mighty Shee Posted December 27, 2019 Author Report Share Posted December 27, 2019 Have you fired this up and seated the rings yet? Did you lube the piston and rings when you were putting it together? Did it have a factory style head gasket when you took it apart? What were the base gaskets-stock thickness or something different? I have never been fond of the Cometic stuff, if I am building a stock type motor with just a cleanup bore/hone I prefer to run genuine Yamaha gaskets. Is there more to this build you are not telling us-high compression head or big bore or stroker or did it have some other specialty piston that raise compression? Was it carboned up when you took it apart- if it had enough piston slap to be noticeable, I wouldn't think it would make the kind of compression that it had if everything was normal.I lubed the ringsI lubed the cylinder wallsI lubed the top of the pistonsI lubed the side of the pistons with the residual lube left on my fingers from lubing everything elseThe motor was using Cometic before the rebuild and I put in Cometic after the rebuild. Before was 65.5mm kit part number C7094, after is 68mm kit part number C7316.The quad already had a milled head and was/is a 4 mil before the buildNothing other than a hone was done. Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
Mighty Shee Posted December 27, 2019 Author Report Share Posted December 27, 2019 I have not fired her up. I was thinking I should probably take it to the shop that did the hone before I fired her up. Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
375hp banshees Posted December 27, 2019 Report Share Posted December 27, 2019 What are old & new piston #'s So they honed it from 65.5 to 68? Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted December 28, 2019 Report Share Posted December 28, 2019 Fire it up and seat the rings. Then check it. Quote Link to comment Share on other sites More sharing options...
Ayesully810 Posted December 28, 2019 Report Share Posted December 28, 2019 (edited) 9 hours ago, Mighty Shee said: I lubed the rings I lubed the cylinder walls I lubed the top of the pistons I lubed the side of the pistons with the residual lube left on my fingers from lubing everything else The motor was using Cometic before the rebuild and I put in Cometic after the rebuild. Before was 65.5mm kit part number C7094, after is 68mm kit part number C7316. The quad already had a milled head and was/is a 4 mil before the build Nothing other than a hone was done. Sent from my iPhone using Tapatalk how did you go from a 65.5 pistons (basically 100 over pistons) to a 68 mm piston? did he put big bore sleeves in your motor after as well? even if he did that still doesnt make sence because youd still be less than 68mm unless they start at 68mm.. dont count on ring seating netting you another 40 psi Edited December 28, 2019 by Ayesully810 Quote Link to comment Share on other sites More sharing options...
Ayesully810 Posted December 28, 2019 Report Share Posted December 28, 2019 (edited) edit. I just read your post again, im assuming now by 68mm you meant the GASKET kit. the only thing that could effect your compression is the base gasket. your base gasket would have to be a hell of a lot thicker to drop 40 psi. a thinner base gasket is going to bring the piston closer to the dome which tightens the squish and increases compression (not significantly though) the opposite is true for thicker gaskets. going from a .020 to a .032 gasket wouldnt drop compression by 40 psi for example Edited December 28, 2019 by Ayesully810 Quote Link to comment Share on other sites More sharing options...
Mighty Shee Posted December 28, 2019 Author Report Share Posted December 28, 2019 edit. I just read your post again, im assuming now by 68mm you meant the GASKET kit. the only thing that could effect your compression is the base gasket. your base gasket would have to be a hell of a lot thicker to drop 40 psi. a thinner base gasket is going to bring the piston closer to the dome which tightens the squish and increases compression (not significantly though) the opposite is true for thicker gaskets. going from a .020 to a .032 gasket wouldnt drop compression by 40 psi for example Yes, I was talking about the gasket when I said 68mm. The guy at Cometic did say that if my bore is not 68mm I may be losing compression by using them. Unfortunately the shop will not open till Monday and I’m heading out of town on Sunday and won’t be able to go to the shop till late next week. At this point I’m thinking it may be the base gaskets at 68mm are too big for the cylinders and that’s what causing my loss of compression. I’ll post my finding here once I correct my issue for everyone else to see. Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted December 28, 2019 Report Share Posted December 28, 2019 Going from 160+ down to 120+ is 40psi. Typically when doing stock cylinder stuff is about 1cc per 10psi. So you’re talking about roughly 4cc worth of gasket material making that difference. I just don’t see the base gasket cut making up that much volume. Plus, the piston crown is never even going below the base gasket for its volume to even matter. Thickness would be the only difference that could cause those numbers to change. Dont waste the guys time at the shop with such a silly assumption. Figure out what the base gasket thickness is and head gasket thickness is. And fire it up and seat the rings before making any accusations at the shop. Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted December 28, 2019 Report Share Posted December 28, 2019 Not sure what your issue is, but in all the years of owning Banshee junk, Pro X pistons were the only ones we've ever had literally fall apart. Quote Link to comment Share on other sites More sharing options...
Mighty Shee Posted January 18, 2020 Author Report Share Posted January 18, 2020 Found the issue to the loss of compression. Went back to the shop with my old pistons and my quad. After scratching their heads over this they finally Figured out the old pistons in my Banshee (Wossner) were taller measured from the piston pin to the crown vs The Pro-X BLASTER pistons they sold me. Why the hell did CT Racing did not tell me this from the get go? Had I known this from the get go I would have insisted on Wossner pistons or Wiseco pistons. They instead sold me what they could get their hands on the quickest. To gain compression they had to mill the head even more. Now I’m at 155 psi, not the 165 I used to have. Sent from my iPhone using Tapatalk 1 Quote Link to comment Share on other sites More sharing options...
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