Power Head for the Pilot By hoser You might not remember the reason why I was looking for a better head for my Pilot, the current head performs perfectly and produces plenty of power for being the off set dome design it just is not up to snuff in the cooling department and I have had problems with it over heating in the past . The adventure of getting a new head for my Pilot is been aggravating at the least, I ordered one from Pro-Design in November of 2001 here it is 3-16-02 and still no head I will not get into all the details but Tom Durba of Epic Designs (guy who makes the stuff for Pro-Design) has won the MUTT of the year award and I wish you good luck if you ever have to do business with the lying SOB, nuff said about that mutt. On the last Little Sahara trip I found out that Duncan Racing makes a Power Head for the Pilot so when I got back I ordered one, they asked a bunch of general questions about my setup then I gave the lady taking my order the following additional information about my Pilot. Current head is 49cc Squish thickness on head is .047 Head gasket .025 0.000 Deck Bowl diameter of head is 2.360 2nd oversize piston 81mm static compression was 170 psi Distance from the spark plug shoulder to the sealing surface of the head is .927 Bowl diameter is 2.36 Diameter of the bore in head is 3.170 Squish ban width is .430 Exhaust port opens at 1.703 distance between the edge of the piston to the top of the cylinder. Exhaust opens at 90 degrees Transfers open at 115 degrees and close at 242 Exhaust closes at 272 degrees. 39mm PWK ATV Racing Rev pipe and quiet silencer. Power Bloc clutch Modified 88 CR250 reed cage. I also told the lady to call me if there is any questions. Well my head came in promptly and as promised (great service) out of the box and looking at the dome I was shocked to see their was no squish cut into the dome. I decided to take it out to the shop so I could cc the dome, right on the money at 49cc so using a old head gasket with the black stuff scraped off I installed the head to take a static pressure reading, 155 lbs darn it was suppose to be 165 my old head gave me 170 lbs but I wanted to drop it back to 165 so I don't have to worry about detonation problems. I intended on buying a second dome with lower compression for desert racing (if I ever make it to one of the races ) and for the Florida rides where we are close to sea level and the fuel quality. Not having the proper clearance between the dome of the piston and the head can be catastrophic, say for instance your running up a hill WOT at the dunes and the clutch cover comes off your engine and unloads the engine instantly without a load on the engine the engine rev's to unknown RPM's before you can get off the gas, you need the clearance to not only prevent detonation you also need it to keep parts from coming together and hitting each other, .025 measured cold is just not enough! The angle of the squish is 13 degrees the crown on the Honda piston is 12 degrees Here are some measurements I took from the new head Bore diameter is 3.182 in Bowl diameter is 2.255 in Squish ban width is .460 Bowl depth is .930 (from gasket surface to the bottom of the bowl next to the spark plug hole) Below is my old head you can see the .047 depth cut into the head add the .025 for the thickness of the head gasket and I was running .072 squish with my old head the rule of thumb is Stroke x 0.0135 = Squish so taking my Pilot stroke at 79mm x 0.0135 = 1.0665 mm then convert from metric to inches my squish should have been .04199 inches or .042 squish, real conservative huh When my head was damaged and the head needed to be cut again we did not know the actual thickness of the dome so playing it safe we did not remove too much material from the dome area, if you remove too much the dome will warp and crack, you could measure the thickness of the squish ban area so removed material there. Time to call Duncan and see what they have to say after all they are in the business and made the head, I talked to loren he said he has never cut a step into one of his heads yet and suggested I do the solder squish test, this is where you lay a piece of solder .100 thick parallel to the wrist pin (so the piston can not rock in the bore and affect test) on the top of the piston using something like silicone sealer to keep it in place while you torque the head once at full torque you gently roll the engine up and past top dead center to crush the solder, the first time I tried it with .100 thick solder it would not go past TDC so I had to remove the head and pre crush the solder to .060 and install the head a second time, this time it went over, it is recommended that you run it past top dead center a few times so I did, now you remove the head and measure the thickness of the solder, you will want to find the smallest number, since I started with .060 thick solder and the largest I measured was .044 thick I know the solder had plenty of crush at the tips so the ends of the solder was measured, the tips were .043 and .044 the closest part to the dome measured the smallest, I found them to be .038 and .041 thick. Martin set my squish in his design at 1.2mm or .0472" with my smallest number being .038 I can see I need to remove some material from the squish area of the dome, I would like to separate the the Honda 3 piece gasket and just one layer so I can get 2 gaskets out of one, with one layer being .010 thick I will need to remove another .020 from the squish area of the dome to provide clearance, I definitely need to increase the squish angle so I will cut using the .041 thick measurement and do the squish test again to confirm the corrected thickness. Below you can see how I laid the solder on the head, some suggest cutting 1" long pieces and using clay to hold them into place, I like one long piece so I do not have to worry about the 1" piece working its way into the bottom end, I didn't have any clay so use a little RVT to hold it into place, you can build a loop and thread it into the spark plug hole to perform this test but removing the head is the most accurate way of getting this measurement because you are 100% sure the tips of the solder are reaching the outer edges of the cylinder. I used a old head gasket to perform the test. After you remove the dome you can measure its thickness, the ideal and safe setup is for the solder to get thicker as you go from the end of the solder towards the center of the cylinder I had just the opposite, Martin says this condition can show a increase in power in a race engine with race fuel but could create problems on a pump gas engine like mine, I will increase the angle from the edge of the bore to the bowl to reduce the risk. Here is the head after removing material from the squish to adjust the angle and to provide more clearance for running one layer of the head gasket, with new solder I now have .046 at the outside edge of the cylinder and .050 right at the edge of the of where the squish meets the bowl, the angle ended up being 14.5 degrees. You can now see a step that I cut into the head adding the proper clearance and allowing me to use only one layer of the Honda head gasket. What I did was put the dome into the lathe and I found the angle of the Duncan squish then set the cutter 1 degree more starting form the inside edge of the squish moving to the outside I made the cut by time I got to the outside edge the cutter was not hitting any more I then removed .020 to compensate for using only one layer of the head gasket the 2 layers not used actually measure .016 thick so I removing .020 from the squish ban I have plenty of margin. With the head installed I have a static compression of 155 lbs about 10 psi less than I wanted, I will try this head to see how it performs buy will probably order another dome cut to Sainty's (Martin Saint of Stealth-Engineering.) specifications, here is the head he has design for my Pilot and I will be ordering from Duncan racing as my second test dome Thanks again Sainty "see how it performs" you read that above and I have some theories I will be testing this season, for years I have been told by lots of different people from professional engine builders to bench racers that the stock off set head design is costing me power, I have to agree it sounds good but lets look a little closer at what is happening, remember these are my theories, when the piston comes up to top dead center using the stock head the air/fuel mixture is being pushed and compressed into the the dome of the head, being located at the rear of the cylinder this puts more load on one side of the cylinder (trying to rock the piston in the bore) then when the mixture is ignited same thing the big bang is distributing more load to one side of the piston or pressure is pressure inside the cylinder and the forces of compression and combustion are distributed evenly across the crown of the piston? With the dome now centered over the center of the bore, this being a better more efficient design in theory I will make more power than with the old head, now will I make promising amounts of power even with having almost 15 psi less static compression than the old head had? Cant answer these questions until I do some testing, "Though I do adhere to the philosophy (or science) that for every action there is an equal and opposite reaction. The problem is figuring out what the reaction is." I hope I am going in the right direction.
Update 5-12-02 Went to Little Sahara for some real testing, after getting out on the trails right away I noticed my engine had lost a lot of the midrange punch it had with my old head and seem to rev out a little slower than before, off idle throttle response was also reduced I will order another dome and increase compression slightly, when Rick originally cut my stock head it had 164 psi static compression and ran perfect, after the crank bearings went out and damaged the head the head was repaired and the compression was 170 since I use pump gas I rather keep the compression down to the point where I am not right on the edge, I will save the current dome for areas of low elevation like Florida and areas where I can not carry enough gas from home to last the whole trip and will be buying gas from a unknown gas station like the upcoming Glamis trip in early 2003. More...