Butcha 111 Posted July 4, 2014 Long story..I have had a problem previously with a head on my speedway engine where the valves stop sealing due to valve seat movement and when I got the machinist to put new seats in and after drilling out the seats he rang me and said bin it cause theres big crack behind the seats which spread from intake and exhaust. Fast forward a bit and recently I removed a c2 head cause of no cyld pressure and found 3 bent intake valves (reason unknown) however I put a good valve in and lapped it and it came up ok so I guess the seat hasnt moved but I can see a hairline going between intake an exhaust on 3 cylders (2 are different cyld with the bent valve) so im guessing its cracked behind the seat as well. Is this a common thing? Would it simply be just a matter of tig welding the cracks recutting and pressing in a new seat? Or just bin it and find another one. Share this post Link to post Share on other sites
slydog 7,873 Posted July 4, 2014 Lol seen heaps of with cracks and they were all on going engines with no issuses other than not enough HP, hence the engine pull down build lol. 1 Butcha reacted to this Share this post Link to post Share on other sites
gerg 10,871 Posted July 4, 2014 Alloy is a wonderful thing to be able to repair. I've seen a bloke resurrect a Suzuki drag bike engine that had Chernobyled itself with a valve head breaking off and punching into the head. Pulled all the steel bits out, built up the mess with fresh alloy and ground back and re-machined it like a bought one. Share this post Link to post Share on other sites
Mr Polson 10,214 Posted July 4, 2014 Very common on even stock motors that don't see high revs or hard work. Every xflow head I've had off except one (removed about 5 in total) have had it, and they were all working fine before dismantling. Share this post Link to post Share on other sites
ando76 4,354 Posted July 4, 2014 Butch I've got one here that is about to go on BITZA and it has a crack between the intake and exhaust seat. had the head pressure tested and no leak. I just peened the crack over to make me fell better and I will be running it like that. I'll see what happens but I'm not expecting any issues. There doesn't seem to be much material there so I am guessing that is the reason they crack there?? Share this post Link to post Share on other sites
PRO250 1,506 Posted July 5, 2014 Just check it over pressure test it and if it checks out use it Share this post Link to post Share on other sites
Butcha 111 Posted July 5, 2014 Cheers fellas thought I was a bit hard on the gear. Was going to send that head off to get .060 or so off it slap som valves in it and put it back on for a bit of cheap hp for me spare engine. Share this post Link to post Share on other sites
ando76 4,354 Posted July 5, 2014 be careful taking too much off them butch. .060 off a virgin head would be max I would go. Share this post Link to post Share on other sites
gerg 10,871 Posted July 5, 2014 There doesn't seem to be much material there so I am guessing that is the reason they crack there?? Yeah that's pretty much it, that and the fact that the alloy and the steel seats have different expansion rates from heat. Such heating/cooling cycles would cause the pressed-in seats to stress the surrounding alloy to the point where the weakest section gives out. 2 slydog and Butcha reacted to this Share this post Link to post Share on other sites
Butcha 111 Posted July 5, 2014 Current engine has pistons .100 " down the bore with a c1 head (50cc) which gives me 9.6:1 not real flash runs ok on methanol tho. I figure .060" off will give me maybe 36cc on c2 head¿ around 12.5:1 compression.. gotta be worth a bit eh? Would certainly get the car off the line a bit quicker. Share this post Link to post Share on other sites
ando76 4,354 Posted July 5, 2014 are you sure about that measurement on deck height butch. Around .050 is usual for stock pistons in my experience. You haven't got a banana rod in there by chance? Why not machine the C1 head. they flow better off the bottom than a C2 head, unless you unshroud the valve and remove the sharp edge and then you end up with a c2 head that has around 47cc anyway. Also remember stock pistons are only rated to 10:1 max so 12.5 might be a stretch on reliability. I would be aiming for around 11:1 and get a bit more timing in there. Did you hurt the big clevo? Share this post Link to post Share on other sites
Butcha 111 Posted July 6, 2014 The engine was built about 8 years ago with 200 rods and flatops with a c2 head on petrol the deck has also been oringed. The machinist knocked the tops off the piston so the engine ran on petrol but was a little too enthusiastic I think. Acl race series pistons with only static balance. Crow hydrualic cam cant remember specs but turns to bit over 6 down the straights and spins up to 68 when you kick it into a corner. Standard valves with steel retainers for dual springs. Dont want to spend a heap on the engine just after easy hp. Snapped a rod in the v8 whist I was grabbing the flag for my victory lap last year. Short engine Has since been repaired and is on the shed floor awaiting top en assembly but I want to put it in me street xe if I ever finish it. Share this post Link to post Share on other sites
ando76 4,354 Posted July 6, 2014 ok cool - now I understand Share this post Link to post Share on other sites
