drew24
New member
Just pm'd you these but I will post them too. Pto 2.850, center 2.820, mag 2.863. 73mm is 2.874. Piston was definitely hitting the center and I think they were kissing on the other 2.
Going to need help setting up 780 srx
- Thread starter drew24
- Start date
1.)looking at the pic with heads upside down, the center head appears to be a smaller dome from what the pic looks like on my monitor, I don't know if its just my eyes or?? if you flip the heads over are they all the same number on them??, will say either 8dn00 or 8dn10.
2.) the bore of the heads is incorrect for the big bore. your diameter of the heads should be about .003-.005" larger then the bore diameter.
A.) theres a couple different options to make it work, 1 is to use extra base gaskets on the outer 2 cylinders, total of 2 on each outside cylinder with 3 total on the center cylinder, this is NOT the right way to do it but it will get the job done and the piston will not hit the head anymore, however the head still isn't too the right bore diameter. (its the least optimum fix but.. it WILL work.) you just cut a stock base gasket with sheet metal shears and stack them up as since you need more in center cylinder you cant keep it whole.
B.) second option is recut and rechamber all 3 heads to suit each cylinder and they would need to be marked and kept on that cylinder since the cylinders are different height. Your squish numbers are gonna be flawed due to the bore of the heads being smaller then the bore diameter so your solder is made smaller by the step in the head, but you have a good .005" error window to use that it wont make much difference. By opening up the diameter of the heads and rechambering you can get the compression around 140-145lbs and it will live fine on premum pumpgas and not give you reliability issues, much above that cranking compression and your walking a thin line with todays pumpgas and a thin line on fuel quality biting you in the butt at a trail fuel stop.
What would need done is too find out which head is the thinnest, dust it off to be true and then cut all 3 to match. Then recut the squishbands so they would all yield the same clearance and angle and radius. If any of the cylinders are a different deck height that head must remain with that cylinder if it needs a different spec to be the same spec as the other 2 are. I stamp heads and cylinders when encountering this.
Lastly, can you post a pic of each cylinder with the piston all the way down at BDC so we can observe where the crown edge is compared to the exhaust port bottom, need the pic to be from rear facing the front of sled.
2.) the bore of the heads is incorrect for the big bore. your diameter of the heads should be about .003-.005" larger then the bore diameter.
A.) theres a couple different options to make it work, 1 is to use extra base gaskets on the outer 2 cylinders, total of 2 on each outside cylinder with 3 total on the center cylinder, this is NOT the right way to do it but it will get the job done and the piston will not hit the head anymore, however the head still isn't too the right bore diameter. (its the least optimum fix but.. it WILL work.) you just cut a stock base gasket with sheet metal shears and stack them up as since you need more in center cylinder you cant keep it whole.
B.) second option is recut and rechamber all 3 heads to suit each cylinder and they would need to be marked and kept on that cylinder since the cylinders are different height. Your squish numbers are gonna be flawed due to the bore of the heads being smaller then the bore diameter so your solder is made smaller by the step in the head, but you have a good .005" error window to use that it wont make much difference. By opening up the diameter of the heads and rechambering you can get the compression around 140-145lbs and it will live fine on premum pumpgas and not give you reliability issues, much above that cranking compression and your walking a thin line with todays pumpgas and a thin line on fuel quality biting you in the butt at a trail fuel stop.
What would need done is too find out which head is the thinnest, dust it off to be true and then cut all 3 to match. Then recut the squishbands so they would all yield the same clearance and angle and radius. If any of the cylinders are a different deck height that head must remain with that cylinder if it needs a different spec to be the same spec as the other 2 are. I stamp heads and cylinders when encountering this.
Lastly, can you post a pic of each cylinder with the piston all the way down at BDC so we can observe where the crown edge is compared to the exhaust port bottom, need the pic to be from rear facing the front of sled.
drew24
New member
is this the pic you are looking for? I can get the other 2 if it is. They appear to be the same. I am going to guess the pistons are about .030 higher than the bottom of the exhaust port. That's a rough estimate. I can take the exhaust off and get a pic through the port if that would help. The middle cylinder head is a 8df-00 head and the other 2 are 8dn-00.
staggs65
Moderator
So that 1 head started out as a 600 head. I have a 700 head here if it's easier to work with that to get it straightened out
drew24
New member
It's weird since I have some stock 8dn-00 which are 2.747 which is .030 over the stock 69mm bore and these heads are cut bigger than stock but not enough for the 780.
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my eyes didn't deceive me then, that's also why the center has higher compression because that's a 600 srx head which is a smaller dome......lol
at this point, I think the easiet route to fix this is to just start over on the heads, all 3 same model and recut a set to suit the engine. I would install a extra base gasket on this engine and then set up the heads to suit, problem over and it will run fine and be good to go. Let me look and see what all I have on the shelf in the morning, I likely have some 98-99 8dn00 heads, you can have one if I have them.
at this point, I think the easiet route to fix this is to just start over on the heads, all 3 same model and recut a set to suit the engine. I would install a extra base gasket on this engine and then set up the heads to suit, problem over and it will run fine and be good to go. Let me look and see what all I have on the shelf in the morning, I likely have some 98-99 8dn00 heads, you can have one if I have them.
drew24
New member
You have good eyes. I didn't notice it. I just held a caliper and eyeballed the edge of the dome and its definitely smaller. I knew that was a 600 head, I just figured it was machined to match. Measuring a stock head vs these they shaved .040 off the heads. I have a full set of 8dn-00 if needed.
Staggs contacted me and explained the situation, and asked if I would repair it for you. I will do so, like to get your phone number so I can call you and explain what all I need informationwise to do the job. Please pm me your phone number and best time to reach you and I will call you.
By the way, that's how nice of a person Jeff is, when he bought something and never used it, sold it and then wants to make sure that something is right on his own dime, that's a good seller/person!
By the way, that's how nice of a person Jeff is, when he bought something and never used it, sold it and then wants to make sure that something is right on his own dime, that's a good seller/person!
drew24
New member
I totally agree. You guys have both been great. Thanks to you both.
Mac
Member
Nice Staggs. That's awesome. Drew gets a an early Christmas gift.
BTW - Mr Viper set up my Mega power 780 heads. And it runs great.
Merry Christmas guys.
BTW - Mr Viper set up my Mega power 780 heads. And it runs great.
Merry Christmas guys.
drew24
New member
A big thanks to mrviper and staggs for getting these heads fixed up for me. They look awesome and all 3 are the same now. I put the 700 top end back on for a little while as I had a trip north last weekend and I am going to leave it for another trip and then if we get some snow here so I can do some testing I will put the 780 top end back on. I had to run with some skidoos this weekend and the 700 did pretty good so I can't wait to try that again as a 780. I'll post when I get it switched back over and start more testing. A big thanks to everyone that has helped me so far and another thanks to mrviper and staggs. Can't beat the excellent service I got from both.
drew24
New member
It's looking more and more like the 780 is going to have to wait till next year. We haven't gotten any snow since December and the forecast doesn't look good for snow. I was planning to go north next weekend and was going to put it on and do some testing but it looks like the weather isn't going to cooperate up there either. This winter pretty much sucks. I might put it on just to make sure everything is good and then fog it and call it good. Maybe get lucky and get one more good snow. Thanks to everyone who has helped on this so far. I appreciate it.
Any updates?
drew24
New member
Not yet. Been too busy. Mr viper did the heads last year and its been sitting on the shelf ever since. I put the temp gauge on last year and was running it with the stock 700 top end and noticed it was running really hot on the trails so I am kind of nervous to put it on and take it on a trip until I get that figured out. I bought some ice scratchers to put on so hopefully that will help that. My perfect plan would be to get another srx to put the 780 on so I can leave the 700 for my trail sled. I have a trip coming up towards the beginning of February so I might put it on after that.
Srx007
Member
Might want to check the thermostat, noticed my srx was running unusually hot, ended up being the thermostat collapsed and was stuck only 1/4 open.
drew24
New member
I was going to check it out again. The funny thing is I was just in there to put the sensor for the gauge in right before the first trip and I already changed it once due to the old one collapsing. Had the same problem the second trip. I got home from each trip and found my reservoir overfull and had to bleed the rear heat exchanger. I think the coolant was boiling and causing an air pocket in the rear exchanger. We got a good dumping of lake effect snow at the end of the season and I ran it all day and had no problems in the deep snow. On the first trip when I was reading hot temps my buddies on there new skidoos kept having there temp lights come on too so I was thinking it was just bad snow. Not sure but I will definitely check to make sure before I take it out and put the scratchers on.
Srx007
Member
What kind of mix ratio are you using, and if there is air in the system you will over heat, and I take it you drained the cooling entire system when you did your work.
drew24
New member
I am running 60/40 now. I know the air couldn't have been there from the start since the reservoir would show it was overfull. Then I bleed it and it would go right back to the full line. I think it was just bad snow. I never had any problems when there was a little fresh powder. Last year's weather was terrible for riding.
drew24
New member
Put the 780 back on last night. A big thanks to staggs and mrviper for getting the heads fixed up for me. Compression is at 153 psi give or take 2 psi. Squish is at .050. I have 45 pilots, 155 mains, fuel screws at 1 3/4, and needles at 3.5, and stock airbox. I rode it for 20 miles last night and it ran great. Rpms were a little low at 8100 but it was pulling hard. I am going to get out in a little bit and do some runs to check wash. The wash did look good after the 20 miles. It was also 25 degrees and its not much colder right now so I would like to check it when it is colder too. Then I can get the clutching dialed in. Clutching is 8bu-00 with .8 inner hole and 4.5 outer hole, G-W-G primary spring, green dot secondary at 70, and advent-edge 50/40 helix. I might try a couple engagement shims to see what that does. It was engaging a little lower than I would like.