Wildcardracing Posted November 13, 2008 Report Share Posted November 13, 2008 When you pulled the spark plugs out, I suspect they looked grey/silver from the aluminum. Detonation will usually erode towards the exhaust port, so you may be able to get an idea of the piston condition by looking through the port with a flashlight. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 13, 2008 Report Share Posted November 13, 2008 This is why you always do a compression check brand new, without even firing the bike. I'd bet the pistons are fine....one of a few things: 1. Piston to wall clearance is out of spec 2. The domes aren't the right size (larger than 17cc, like a 21 or 22cc) In that case...it's fine 3. BOTH compression testers are wrong, or you did the test incorrectly... While I agree 17cc should need race fuel, 110 all the way....I would find it very hard to believe you damaged both pistons THE SAME and it's equally as low on both... I'm leaning heavily towards 2...with a little 3 thrown in for good measure. .055 is a little big for a small bore motor, but...it should produce a LOT more than 135 if it were 17cc domes. I run .055 in my motor, but I also have a bigger bore, more timing, stroked crank and I run alky on an aftermarket cylinder...with 180 to 185PSI on 20cc domes. Before you worry too much, drain the coolant by removing the lower rad hose from the water pump cover, and pull the head off. There will be an arrow on top the pistons, they need to be pointing toward the exhaust. Quote Link to comment Share on other sites More sharing options...
desertdeweller Posted November 13, 2008 Report Share Posted November 13, 2008 when you first started it how did the compression feel was it easy to kick over or did it feel like it had a lot of compression. I wonder if they gave him long rod or stroker pistons Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 13, 2008 Report Share Posted November 13, 2008 when you first started it how did the compression feel was it easy to kick over or did it feel like it had a lot of compression.I wonder if they gave him long rod or stroker pistons He wouldn't be at .055 squish if that was the case.... Stroker pistons would make his squish over 2mm big, long rod pistons 5mm or more larger... .055 is close to where he needs to be. Quote Link to comment Share on other sites More sharing options...
desertdeweller Posted November 13, 2008 Report Share Posted November 13, 2008 true 2 mmwould be around .090. hmm is he running a head gasket with the cool head? Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 13, 2008 Report Share Posted November 13, 2008 And a long rod piston would be in the neighborhood of .170....give or take depending on the base gasket thickness.... Good point....hope he's not running a head gasket in addition to the Orings. I think he needs to cc the domes for himself. What is the part number on tops of the pistons, that could solve the stroker/long rod piston dillema... Quote Link to comment Share on other sites More sharing options...
craptorkiller Posted November 14, 2008 Author Report Share Posted November 14, 2008 And a long rod piston would be in the neighborhood of .170....give or take depending on the base gasket thickness.... Good point....hope he's not running a head gasket in addition to the Orings. I think he needs to cc the domes for himself. What is the part number on tops of the pistons, that could solve the stroker/long rod piston dillema... its a stock stroke long rod crank. it has the 513 pistons in it. i think i found the problem. the ring didnt seat properly and l left a gouge right next the exhuast. but that was only one cylinder? so i still dont know why it would be the same in both cylinders? im just using the orings. how do you check the cc's? Quote Link to comment Share on other sites More sharing options...
craptorkiller Posted November 14, 2008 Author Report Share Posted November 14, 2008 When you pulled the spark plugs out, I suspect they looked grey/silver from the aluminum. Detonation will usually erode towards the exhaust port, so you may be able to get an idea of the piston condition by looking through the port with a flashlight. luckily i didnt see any damage to the pistons!! Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 14, 2008 Report Share Posted November 14, 2008 Take a piece of Plexiglas, drill a small hole in the center. Use grease, oil, etc.....seal the glass against the dome. Plug it with the spark plug. Use a syringe and fill the dome through the hole in the Plexiglas until it is full...see how much water it takes to fill the dome flat/even to the Plexiglas, that's how many cc's the dome is. Obviously, you have to use a syringe that is graduated in cc's. Quote Link to comment Share on other sites More sharing options...
bbcmudtruck Posted November 14, 2008 Report Share Posted November 14, 2008 The domes should say right on the side. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 14, 2008 Report Share Posted November 14, 2008 (edited) The domes should say right on the side. True. But... A. He's fighting VERY low compression. SO....manually cc'ing the domes is necessary at this point B. Some domes aren't stamped, or stamped incorrectly... NEVER take stuff like that for granted. Edited November 14, 2008 by dajogejr Quote Link to comment Share on other sites More sharing options...
bbcmudtruck Posted November 14, 2008 Report Share Posted November 14, 2008 I agree however if he pulls the domes and they say 21cc right on the side, (not 17), then he wouldn't have to mess with manually cc'ing. 135 with 21s would be about right. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 14, 2008 Report Share Posted November 14, 2008 True. But what if he pulls them, they say 21 and cc out to 19...or they cc out at 17cc? Or...they have 17 stamped in them, but cc out to 21?? Often times...in haste, builders take a dome and re machine it, don't change the stamp on the side...and at this point, it is in his best interest to get the actual cc reading, not what is stamped on the side... This is a great learning exercise to not only understand how things work, but to also know how to measure something properly... Quote Link to comment Share on other sites More sharing options...
bbcmudtruck Posted November 14, 2008 Report Share Posted November 14, 2008 Do you put the oring on the dome, or just sit the plexiglass on it? Quote Link to comment Share on other sites More sharing options...
dajogejr Posted November 14, 2008 Report Share Posted November 14, 2008 Just plexiglas, but you need to use a sealer of some type like grease, etc., so the water doesn't seep past. The Oring by itself will not seal it...you need something else. Unless you want to put a C Clamp against the dome and plexi.... Quote Link to comment Share on other sites More sharing options...
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