260ZRED Posted March 11, 2007 Share Posted March 11, 2007 ok a large problem. here is the set up. l28 block w/ flat tops e31 head shaved just for suface 2-3 times l28et valves (P90a) sh*t load of port work Before we get to far into this i already know that I used the wrong valves as they are to short. I was told i actually need n42 valves, is this correct. Are they the same dia? here is what I did.... 1. I set the head on the block with no head gasket and no head bolts. 2. set the cam timming. 3. set the lash 4. turn the motor over by hand VERY slowly Well.....after about 1/4 of crank rotation she stoped. tha intak valves contacted the piston. so I pulled the head back off and repeated the previous steps with a brand new head gasked but still did not bolt it down. the same thing happend. So the bottom line is i should have resurched this. DOES ANY ONE HAVE ANY SUGGESTIONS??? I really want to stay away from a thicker head gasket and pistons. I alread have a crap load of money in this motor. but if pistons are my only fix please tell me where I can pistons that will keep my compession around 10.75:1 Thanks Much Quote Link to comment Share on other sites More sharing options...
Phred Posted March 11, 2007 Share Posted March 11, 2007 Before doing anything else, reset the lash in increasing increments of .020 or so. Then recheck. Keep resetting the lash till you have some V-P clr. Once you know how much lash you need to clear the pistons, Then you can dicide which action you must take. if possible check your cam timimg as in the Cam timing sticky by Braap. You need some info that only a dial indicater will provide. Note: The valves will be closest to the piston at 10 degrees before (Exh.), and 10 degrees after ( Int. ) TDC. You will need a Minimum of .040 Int. & .060 Exh. Preferably a bit more. Phred Quote Link to comment Share on other sites More sharing options...
260ZRED Posted March 11, 2007 Author Share Posted March 11, 2007 was actually thinking of stacking head gaskets till there was no more tap tap then pulling the head and put a little putty or clay on top of the piston to get an idea of clearance. then measure the amount of gasket I added, although i am not sure where I would go from there. thanks for the addvice. Quote Link to comment Share on other sites More sharing options...
TONY C Posted March 11, 2007 Share Posted March 11, 2007 worse case you could get a 2mm head gasket from nissan. if that is not enough then get 2. 2mm gaskets and take them apart and use the pieces you need to get what you want. a little pricey but will work. Quote Link to comment Share on other sites More sharing options...
260ZRED Posted March 11, 2007 Author Share Posted March 11, 2007 would work but there goes my 10.75:1 compression ratio Quote Link to comment Share on other sites More sharing options...
technicalninja Posted March 11, 2007 Share Posted March 11, 2007 I might be able to help but need more information. What cam is in it? Specs would be helpfull. Thickness of the head? Really need to know how much was taken off of bottom of head. You can use a dial caliper and measure. Stock heads are 4.248" but as both sides are usually cut during a milling procedure (to retain parallelity) it is nice to know how much was taken off of bottom. Was the block decked? If so how much? The 2 mm shorter valves will not have any effect on piston to valve clearance. They may cause problems with retainer to valve stem seal clearance and they will increase spring loads as the spring is at .080 less installed height. If your using a big cam this interferance will be an issue and could cause you to DROP a valve. This is what is called a catostophic faliure. You need to really make sure this will not happen. The standard L28 valves will work fine for you. 75-80.5 N42, N47 heads. The P series heads P90, P90A, P79 all used the 2 mm shorter valves. You did have a machine shop cut the larger valves into the E31 head? Are they experienced with L-series stuff? Stacking head gaskets in not a good idea IMO. Some people have made it work but.... 2 mm head gaskets are a possible solution but are expensive and reduce quench which a 10.75 comp engine needs. They also reduce compression. The MN47 head is a possible solution but it has the smaller intake valve and I believe it has the valve seat height of the early heads so using it would change nothing in regards to your problem and cost money. Did you degree your cam? If your interferance is small, valve pockets could be cut into pistons but this weakens them and reduces compression slightly. It might also affect the engines balance if much was taken out of the piston. Things that reduce piston to valve clearance. Increasing valve size. You did this. Milling head on bottom. You did this. amount unknown Using a head that has the valve seats 2 mm (Approx .080) closer to the gasket surface. You did this. Decking block- unknown Installed valve depth. IE how deep in the seats were the bigger valves installed. We want them as deep into the seats as they can go and still have a proper outside margin and no recession below chamber surface. increasing lift and duration on cam shaft- unknown Not properly installing cam. At this level of build (this many changes to basic engine geometry) dialing in the cam is more of a requirement than an option. One of my last customer motors was built using a flat top F54/N42 with minor head milling that used a mild Schnieder cam. I intentionally overset the valves till I had valve to piston contact and it was alot closer than I though it would be. Both the intake and the exhaust figures were close to my limits of 0.060" intake and .100" exhaust (clearance between valve and piston at closest point). Two things came to mind. If cam timing was off just a little I would have problems and if I built this with a bigger cam it would require valve pockets in the piston. The cam was the 274-F grind. .480 lift 274 degrees 214 degrees@.050 pretty whimpy IMO but the customer wanted excellent manners in traffic and good fuel economy. as I remember the exhaust clearance was more of an issue that the intake. About 15 degrees before TDC was the closest point. The head was milled the minimum amout to get a clean gasket surface and had been milled once before. I belive it had about .015 total removed from lower surface. This build up mirrors yours in regards to valve/piston placement. Hope this helps Quote Link to comment Share on other sites More sharing options...
260ZRED Posted March 11, 2007 Author Share Posted March 11, 2007 this is excelent info and I am grateful for your time and input but I am a little fuzzy on what you mean by dialing in the cam. I undestand you probably mean finding the proper timming but my cam timming is set stock with a brand new timming set. I am aware the cam sprocket has 1 2 3 possitions but other than that??? what head were they using also? cam is factory for early zx. (dont remember if n42 is head block or both? maybe block was n47?) Quote Link to comment Share on other sites More sharing options...
260ZRED Posted March 11, 2007 Author Share Posted March 11, 2007 ok well i am an idiot as i know see the stick, thanks. Quote Link to comment Share on other sites More sharing options...
v80z Posted March 11, 2007 Share Posted March 11, 2007 Man I hear ya. I cant see how that could happen unless the cam timing was off. If it is not off and it is due to decked block and shaved head it would be nice to know if it where the intake or exhaust. With the decking and shaving do you have alot of slack in your timing chain or have you compensated for that with cam tower shims? Just trying to get a feel for how much material is missing? Does the head still have the reflief marks or has it been surfaced past those? #1 up tdc, lobes at 10 and 2? Quote Link to comment Share on other sites More sharing options...
260ZRED Posted March 11, 2007 Author Share Posted March 11, 2007 Intake. cam i timed perfectly stock. Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted March 21, 2007 Share Posted March 21, 2007 with that kind of build cam timing should be checked and verified with a degree wheel on the crank-verified against cam card packed with cam. Quote Link to comment Share on other sites More sharing options...
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