High compression mods to cylinder head

crmachineman

crmachineman

New Member
Hi everybody. I just wanted to share with you all some of the details about the experimenting I've been doing. Over the last couple of days, I've been testing a modified cylinder head. The engine was a GT-5 skyhawk engine which had a number of problems affecting the power output of the engine. First, the transfer ports were very restricted, so I carved/beveled channels in the piston crown. That didn't quite seem enough, so I put an additional gasket on the bottom of the jug. I also "decked" the jug, taking about 1/16" inch off the top of the jug. Then I put the head in the lathe and faced some material off, and created a squish band. I am very happy with this low cost modification. With proper fuel quantity, detonation is kept at bay, and the power increase was very noticeable! The nice thing about taking the material off the top of the jug is that I can lower the compression by simply adding another gasket between the jug and head. Now I am about to experiment with the older style slant head, and I'll report how that goes. After all this experimenting, I will be able to make a brand new head, machined from solid round stock. I am torn however; do I work within the limitations imposed by the original studs, or should I design the head to be used with longer studs?Since I have ample room above the engine, I am leaning on getting longer studs, so I don't have to recess the retaining nuts deep in a counterbore. I know I won't be able to sell heads for less, or even the same price as Pirate cycles, but I think I can make them reasonably priced. The head design will be round instead of square. I'll cut the cooling grooves in the lathe which will be very fast. I'll use a long reach spark plug as well. I think I'll machine the spark plug hole directly above the cylinder, so It won't be a slant head. Well, cross your fingers, hopefully it works out. I will probably have something to test in a few weeks. I was thinking also about selling some decked jugs, so other people without a machine shop will be able to deal with the problems I had to address with my GT-5 engine. The modified heads could also be sold for a very low cost, however, I'm still working on the best combustion chamber contours. Sorry about the long winded text here. Let me know what you think..duh.
-Fred
 
I say keep doing what you're doing!
All the great stuff you see for these little bikes came from guys like us tinkering in our garages & eventually selling it! (^)

As far as ideas for you... I would say to make the heads small enough for guys with smaller bike frames to be able to bolt them on their engines too.
 
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wish i had the time.
and machinery
and the skills
i have the patience
my 1947 south bend lathe is a little finnicky for fine work.
keep it up,interesting stuff.
 
Hi Crmachineman, yeah, I would leave the stud length the same, some people have compact instalations, & theres upgraded studs available in the same length. I have an RSE head with cooling fins cut the same way ur planning & cooling isnt an issue & they fit on stock studs. Might b an idea 2 use the same reach plug that everyone is already using 2. Keep up the good work & good luck with it. Cheers
 
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ivan H said:
Hi Crmachineman, yeah, I would leave the stud length the same, some people have compact instalations, & theres upgraded studs available in the same length. I have an RSE head with cooling fins cut the same way ur planning & cooling isnt an issue & they fit on stock studs. Might b an idea 2 use the same reach plug that everyone is already using 2. Keep up the good work & good luck with it. Cheers
Click to expand...

Right. Well, I just don't want to make too many compromises; short reach plug equals less aluminum. I want to try to pack as much cooling fins as I can. Spark plugs are cheap enough, right? Well, anyway, I'll definitely think about what you said; you have a lot more experience in the motorized bicycle world than I do.
Thanks
-Fred
 
Venice Motor Bikes said:
I say keep doing what you're doing!
All the great stuff you see for these little bikes came from guys like us tinkering in our garages & eventually selling it! (^)

As far as ideas for you... I would say to make the heads small enough for guys with smaller bike frames to be able to bolt them on their engines too.
Click to expand...

Yeah man! Just yesterday, I did a flurry of tests. There seem to be two types of heads for the 66 cc engine. I first started playing with the new GT-5 skyhawk head. I put the squish band in it: That one is a good power producer! I'm happy with the mod. I've done. It's also interesting to note, the cylinder of the GT-5 skyhawk motor has slightly different transfer port shape and location. Now I'm doing tests on the old style head and jug. I'm running into a few difficulties, but I'll get through it. This is really interesting, the old style head runs the engine differently. So far, I have not been able to match the outright power of the newer skyhawk head, but the engine runs noticeably smoother. I think there is a good chance that I'll be able to measure these modifications very precisely, and reproduce them exactly in the form of a kit consisting of a Piston, cylinder, and head. The nice thing about modifying the existing parts from China, is that it will be very cheap for anybody to buy. All the mechanic will have to do is decide whether to run one or two gaskets on the bottom, then depending on that, a thick or thin head gasket. I'm thinking I might put a groove in the bottom face of the head, so we can run a simple copper or aluminum ring as a gasket.
I'll keep everybody posted on these nuances as they bite me in the ass!
-Fredlaff
 
Hey Fred, Sounds like a worthy pursuit! Wish I had access to a machine shop. Did you by chance ever read foureasys thread about all the mods he did? He ended up getting 8HP, at the rear wheel. Using a Puch 47cc cylinder kit married to the HT bottom end! The thread is in this performance section. Almost sounds like you may be following in his footsteps!

Peace, James
 
Hi James, Interesting. If I get some time, I'll look at those threads. 8 HP. is amazing! I'm trying to figure out how much of the exhaust port should be uncovered when the transfer ports begin to open; maybe about 3/16"? or more? There are so many variables, it's difficult to know what the right balance is. I'm noticing differences in the new GT-5 skyhawk engine; one, it has a different piston. The wrist pin location in higher because they chose to use a longer connecting rod. Another thing different is the transfer ports are a little lower and shaped differently. Also, the head is different. I think the skyhawk engine is better, but when I first started running it, I was not happy with the performance. There was a time when I had explosive acceleration with the skyhawk motor, but then I thought I'd fart around more with the dremel, and uhh... duh.... what happened? Ha, Ha! I bet that's happened to you before! Well, at least I remembered what I had done, so now, I'm trying to recreate what I had. Anyway, I've to stop tinkering and start earning some money!
Talk to you later..:-||
-Fred
 
^^ Hey Fred, oh yea I know all to well how easy it is to get carried away with the Dremal tool. Too bad it can't be replaced once it's ground off eh? Oh well at least new parts are cheap with these lol.... Keep up the good work!

Peace, James
 
Hi Crmachineman, there's 2 piston crown heights in these, some have 0.100", some 0.160". The band should b cut 1 degree positive of the crown & squish clearance needs 2 b 0.030" max 2 b effective, down 2 a minimum of 0.020". Squish is measured by placing plastiguage (or solder) on piston crown in squish area, (hold in place with vaseline) directly over piston pin, torquing head down & cycle thru TDC, remove head & measure. Cheers
 
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