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Discussion Starter #1 (Edited)
So it's been a long time in the making but I have begun putting my '93 GSXR-750 back together and I seem to be having some trouble getting the timing right. Got the pistons and cylinder block back on no problem. Put the head and the timing chain on as per my Clymer manual's specifications (TDC with the #3 on the intake and the #2 on the exhaust cam pointing straight up and the #1 on the exhaust cam in line with the top of the head, see pics below). The manual suggest that I should be able to turn the motor over freely in either direction once installed correctly (obviously) however, when I put everything back together I can't turn the engine CW past TDC...:face palm Oddly enough, I assumed that after taking the cams back off that the motor should turn freely (with all of the valves closed) but it still wouldn't go past TDC... Once I take the head back off though, the pistons freely rotate again... Thought maybe I put the valves in backwards but the large valve on the intake side seem to correspond to the large indentions on the intake side of the pistons. Does anyone out there have a suggestion as to what I am doing wrong???
 

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Timing is not your problem. With the cams removed you have none. Look for something obvious. Did you use a thinner head gasket? Did you forget a base gasket?

Looking at the pics, those look like aftermarket pistons and I would bet, if they are, that you have piston to head contact somewhere.
 

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Wrong head gasket? You cannot use the OE head gasket with aftermarket/oversize pistons as the edge of the gasket will hit the pistons. The pistons should also sit approximately 0.5mm below the gasket surface when at TDC. If the block has been machined too much there could be piston to head clearance issues.
 

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Wrong head gasket? You cannot use the OE head gasket with aftermarket/oversize pistons as the edge of the gasket will hit the pistons. The pistons should also sit approximately 0.5mm below the gasket surface when at TDC. If the block has been machined too much there could be piston to head clearance issues.
Good point and may well be just the wrong head gasket.
 

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Discussion Starter #5
I have an aftermarket Cosmetic head gasket that is made specifically for this bore kit so that shouldn't be the issue. I also have the correct base gasket installed as well so that should be fine as well. Any ideas on how I can check the piston clearance?
 

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The correct way would be to clay all four pistons ( put a thin layer of clay, preferably made for the purpose) on the higher areas of the piston where contact is most likely. Or some thin soft solder located with a dab of grease. Turn the motor over two or three times by hand and pull it apart and see where the clay/solder is dented. As you can't turn it over, you'll just turn it as far as it goes, then pull it and look for dents. Did you also cut the deck? (Top of the cylinder block)

Before you clay or use solder, follow Wombat's suggestion. LOOK at the deck when the pistons are TDC ( either pair). You probably will need to get several sleeves and use your head bolts to snug down the cylinder block to compress the base gasket to take up slack. The outer edges of the piston should be below the top of the deck. I don't know on the water cooled, but the later models you can actually see flush or almost flush with the deck. The head gasket provides the clearance. I don't know the squish for your bike so can't do anything more than guess it's probably 35 thou, the later ones are much less. Then measure your head gasket at a three layer area. What you are looking for is whether you have clearance. The rods/crank/bearing slop ( mostly rod stretch) mean that the top of the piston can rise up to 35 thou (or even more on some motors) thus creating a self destructing device if the clearance is too small! If you are hitting the head with the piston when the motor is turned over by hand, something is WAY off.
 

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Discussion Starter #9
Is that the 907 kit? I thought you had to use the gasket from the rf900...????
Good eye, yes it is the 907 kit. I got my gaskets through APE racing, they were both Cosmetic brand gaskets and to the best of my knowledge they are meant for this specific build. I don't believe a gasket for an rf900 would work as the bore for one of those bikes is 73mm and the 907 kit has 77mm pistons.
 

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Make sure none of the head gasket overhangs into the bore . . . steel does not compress at all. BTW, you did have the plugs out, right? Eyeball your setup, it should be obvious what is hitting at TDC.
 

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Also, did you have the head machined to increase the compression? Going overboard may result in the domes touching the head. If you have access to prussian blue put it all over the piston crown on the offending cylinder (or any one if they all touch), assemble the engine and rotate the crank until resistance is felt. Pull the head back off and see what left a blue imprint on the head.
 

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Discussion Starter #12
BTW, you did have the plugs out, right?
Haha yes, the plugs were out of the head and no the head wasn't machined, just resurfaced for proper run out. Luckily I will finally have some free time to look at it over the next few days and see what I can't figure out. :scratch
 

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Discussion Starter #13
Alright guys, I took a look at the bike this morning and this is what I discovered. I took ErnieM's advice and clayed the crown of the pistons (great advice btw!). Turns out the pistons are hitting the cylinder head on the shoulders of the crown (see pictures below, the spots of contact are circled in red). It appears that I have two obvious options, either mill the cylinder head to clear the piston (the better of the two ideas and probably what I will end up doing) or have the pistons milled down. Just wondering if anyone out there has done this build before or knows the proper solution for this problem :dunno
 

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Do it all the time. Place head on barrel with dowels in place so that you can scribe the head with the location of the bores. Remove head from barrels. Attempt to put piston crowns into head using scribe marks as a guide. Get a flared/flame carbide burr in a dremel and remove metal from head until the piston fits into the head completely with the flat sections of the piston touching the head gasket surface. Tidy the rough head finish with a sander of your choice. DO NOT TOUCH THE HEAD GASKET SURFACE WITH THE CARBIDE . . . I use an old head gasket while grinding to protect the head. Bearing blue will make the job easier as you can see what is touching.
 

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Discussion Starter #15
Alright, after a full day in the lab I think I finally resolved the issue :woohoo! Thank you for the advice Wombat, you were definitely spot on! Picked up some prussian blue from a locate parts store and applied it to the piston crowns figure out where it was hitting and milled the head with a carbide bit. Put everything back together and the motor turned over just fine. Thanks for all of your help everyone, I just assume everyone is like me and likes pictures so here ya go! :cheers
 

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