Cylinder head/Fred head

Discussion in 'General Questions' started by barnett256, Nov 9, 2015.

  1. barnett256

    barnett256 New Member

    I'm getting an 80cc 2stroke... I want to replace the ugly stock head with what would be the equivalent to stock Fred head but I don't know what the specs mean or what works with what.
     

  2. jaguar

    jaguar Well-Known Member

  3. butre

    butre Well-Known Member

    the 6cc fred head offers an 11:1 compression ratio and in my opinion is the ideal head for a mildly modified engine running on pump gas. a fred head doesn't just increase compression, it also improves cooling, so if you throw in a colder than stock spark plug (NGK B7HS is what I use) then you may not need to retard your ignition.
     
  4. jaguar

    jaguar Well-Known Member

    well everyone is certainly entitled to their own opinion, even if it's wrong.
    Butre usually you are pretty right on but not in this case.
    an 11:1 ratio will give over 160psi easily, depending on how much the head gasket leaks and how much blowby these cheap rings allow.
    I advise against higher than 135psi compression for 3 reasons:
    1) the upper conrod bearing is not very high quality and will wear fast with that excessive combustion pressure.
    2) excessive cranking pressure causes excessive engine heat, and these engines are limited in that they are air cooled.
    3) increasing the cranking pressure decreases the burn time and so for it to stay in harmony with crank position you have to retard the ignition, otherwise too much pressure is built up and the optimal push against the piston is lost.

    yes, excessive pressure will give more power but that power will be lost as the engine overheats.
    burning.jpg
    Here's a good head temperature gauge: http://www.jnmotorsbikes.com/product_p/jnm1183.htm
    http://www.dragonfly75.com/motorbike/headtemp.html
     
  5. butre

    butre Well-Known Member

    that guy had a major fuel leak and possibly a lean condition. that's a stock low compression straight plug head, compression ratio had nothing to do with that fire. I'd bet money that gasoline had been dripping on his cylinder head the whole time he was moving, and then when he stopped it started dripping on his hot exhaust pipe.

    my old bike had a 6cc fred head with a decked jug and stock ignition and somewhere in the neighborhood of 175 PSI compression and at max operating temperature you could touch the fins on the head without burning yourself. my current bike has a CDI I put together myself that is essentially a ripoff of an HD Lightning, but I also have one of Manic Mechanic's adjustable magnets advancing the timing by 2 degrees. the 5.3cc Fred Head and a B8HS keeps it nice and under 350 degrees.
     
  6. jaguar

    jaguar Well-Known Member

    like I said, an increase in compression requires ignition timing retard with a different CDI.
    really a person needs to monitor his head temp to make sure he hasn't overdone it.
     
  7. barnett256

    barnett256 New Member

    Honesty I'm not educated enough to turn this into a science project. I figured there'd be a better looking head with same as stock specifications (cc's, whatever) that I could bolt on and rock n roll! ? I thought I saw a 5 & or 5.5 on Fred's site. I can't believe there isn't a head same as stock that looks as nice, bolt on without retarded complications! Sheesh
     
  8. butre

    butre Well-Known Member

    The 7 cc head offers a 9.4:1 compression ratio which should serve you well. No fuss, just better performance and better cooling. Just ask Fabian, that's what he uses and nobody runs their bikes like he does. 400+ pound towing at low speeds and high RPMs. A jaguar CDI doesn't hurt either. HD lightning is good too, but if reliability is what you're after the Jag can't be beat.

    I use a bronze top end bushing instead of the original caged roller bearing for improved reliability and because the failure mode isnt quite so catastrophic. I don't have to deal with that junky bearing exploding on me and ruining my expensive billet head (which I bought when they were still $90) and ported jug I put a lot of time into.
     
  9. Arty

    Arty Active Member

    I am running a 6cc Fred Head on my bike, and as I expected, the increased fin area really helped the cooling on my rig. It's really noticeable right after a fairly long run when leaning hard on the throttle. - The engine is nowhere near as hot, and settles down into a proper idle way better. It even sounds different - almost as if you can hear the air circulation around the cooling fins. - - The thing looks great too.
    Also - Fred at CR Machine is really nice to deal with, and provides prompt service.
     
  10. skyash

    skyash Active Member

    At work got my stock slant head and took off 1 mm then re done the face like stock again just been for a ride and it picks up better and tops out better but I can't put a hand on it. So hot so put more oil 30 ml to 500 ml ish of petrol and b7hs ngk plug and the head gasket might have been 1 mm but off another motor it's like paper thin so I used that one may of drop it by 2 mm over all now. all I can say is it goes harder for now how long?? It will be 34deg tomorrow so I will ride it after work think that's a good hot day test lol
     
  11. jaguar

    jaguar Well-Known Member

    of course you can't put your hand on a 400 degree metal surface.
     
  12. skyash

    skyash Active Member

    My head whent really well feels better all the way from bottom to top.5 to 10 kmh better but at 60 65kmh it starts to play up i got a 20inc whith 36 t and maybe iam ringing it out my plug gap is huge but new cdi is priced to hi in Australia what if it doesn't go better with a hi performance one I can get a new kit for $130 after posting the cdi is the same price??? What to do
     
  13. jaguar

    jaguar Well-Known Member

    you might be losing spark at high rpm due to excessive plug gap
     
  14. skyash

    skyash Active Member

    But it goes better with bigger gap. I heard one man say he uses a 8 spark plug and I got a 5 6 and 7 all go good to start you hit 40kmh open them and then 50 open more and 60 some kids I build bikes for have new kits I think like a stock plug open it and thay don't start close it right down and off like a rocket. If I put my 28 t on it needs a pedel to start but then it pulls up to 60 /65 and is not reving hard but just won't get over a flat spot and rev out. Getting new plug today if I can I will make videos of it going so you can see or hear it
     
  15. skyash

    skyash Active Member

    Or maybe I make the speed carby change needle on the go I am one off the bottom but it makes a wet plug when at bottom so when at hi speed it might work and what do you think of the stock muffler empty with a coin at the end or disk that can close or open at will I have one it can go as quiet as stalling on idel or like a Lamborghini when open mine is a steel pipe not the stock one 1inc longer works on a gear changer cable and spring you see cops and turn it down I love it
     
  16. sbest

    sbest Active Member

    Jaguar and Butre and a couple others, you are worth reading. Thanks for being there...

    I did a lot of work on cylinder head chamber shapes and volumes, and rebuilding and modifying 2 stokes on snowmobiles, bikes and most recently Blaster quads, since the 1970s. I've stuffed, timed, cut, filed, ported and swapped. My experience is with 125-250cc engines. Jaguar, I really respect your website and CDI box. Excellent work. I only have limited experience with my pre-used HT motor. I checked and set the squish while getting it reliable with an overall bolt check, but I feel I have to wade in on the head issue.

    First of all, chamber shape is most critical to combustion characteristics, comparatively more than compression ratio. A wide squish band and tight flat pocket combustion chamber support low rpm torque better. The best shape for broad torque is a torroidal chamber that will support a donut shaped tornado of twirling combustion. The squish has to be set (on a 125 anyway) to 0.5 to 1mm max to get this effect. The present HT head supports high rpm power and the squish was off on mine at 1.5mm.

    Jaguar is right about cranking pressure, but head volume, compression ratio and port timing all go hand in hand. Set the port timing highter and you can raise the CR without putting the cranking pressure too high. One balances the other. Compression ratio mainly affects the low rpm and tends to drive heat into the head and piston if too high. Jaguar is right about needing more timing retard at higher rpm. critical. In fact, the more power you make, the better the combustion shape, the less timing advance a 2-stroke needs.

    About plugs, READ THEM. The condition of the the deposits on the insulator near the electrode tip tells you about the plug heat range. First however, you have to have the carb mixture right. That is read on a new plug insulator down next to the shell after a long full throttle run. We sometimes even cut the threads off the plug to see this area to read it. Just looking for a brown soot ring at the base of the insulator.

    Even before that, check for vacuum leaks. 2 strokes are very vulnerable to vacuum leaks. Plug the intake and exhaust and pump up the cylinder and crankcase to 5-7psi with a handpump and see how long it holds it. 3-5 minutes would be a minimum. This is an important test for seals and gaskets.

    Butre, I am interested in this bronze rod bearing idea. We used to use them in our universal joints when drag racing for the same reason. Many old outboard engines used them too, but used lots of oil and low rpm. Modern roller bearings have proven able to stand up, so what is the failure mode? I am guessing a bronze bushing would only be needed to overcome:
    1) poor rod bearing surface hardening
    2) poor pin hardening
    3) poor roller quality
    4) detonation problems
    Thoughts?

    Steve
     
    Last edited: Nov 20, 2015
  17. butre

    butre Well-Known Member

    the typical failure mode is the cage breaking and the rollers falling all over the place. a better bearing can prevent that, but bearing failure is still catastrophic while bushing failure is gradual and doesn't leave a bunch of hardened steel pins floating around in your engine.

    as for the oiling issue, I get around that by simply drilling a hole in the bushing to match the hole already in the connecting rod.

    I've considered rifle drilling the connecting rods as well, but I don't have any bits that long under a quarter inch so I haven't tried it
     
  18. skyash

    skyash Active Member

    Well it goes so good and i am so happy still a big gap in the plug but
     
  19. skyash

    skyash Active Member

    I Put the fake deleto carby on and 65.4kmh 20inc wheels and 36t sporket b7hs 10 ngk I th
     
  20. skyash

    skyash Active Member

    Sorry this phone is ****ing me i think more petrol wood get more out of it but it's been off for 30 Mins and still hot as all he'll but that 1mm off the head is grate may be some fins bolted on to help with the heat and iam done lol for now
     
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