Twister Hammerhead Engine

 

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Twister Hammerhead 150cc Engine

The Twister Hammerhead karts are powered by a 150cc engine based on the Honda GY6 scooter engine design.  The GY6 was originally designed by Honda as a 125cc engine.  These GY6 cloned engines are now offered in 150cc.  The Twister Hammerhead engine utilizes one of these GY6 based 150cc engines.  This small engine does a surprisingly good job of moving such a big kart around.  Sure, a little more power would be nice (when is it every enough!), but considering the small displacement of this GY6 based 150cc engine, it is impressive how it can move the Twister Hammerhead kart around like it does.  The kart itself weighs in at over 500 lbs, so when you add in the weight of a driver and even a passenger, it's possible to be tipping the scales at close to 1000 lbs.  That's a big difference compared to the relatively lightweight scooter that these engines were originally designed to power.  Even with such a heavy load, this little engine does a good  job of carrying this load without complaint.  Below are a few pictures and some comments about the Twister Hammerhead engine design. 

GY6 Engine with Cooling Shrouds Removed

Honda GY6 Engine Design - Head & Cylinder Exposed

One of the first things that is noticeable underneath the valve cover is that the rocker arms appeared to be high strength forged parts.  This is apparent because of the surface finish on the rocker arms which look to be formed from the forging process.  The cylinder head has an overhead cam with a hemispherical combustion chamber with 2 valves.  The die cast aluminum cylinder looks very nice, and it has a cast iron cylinder sleeve pressed in which should give the GY6 engine long life potential as far as cylinder bore wear is concerned.  The head gasket is a thin metal design. 


Honda GY6 Engine with Head Removed

GY6 Engine with Cylinder Head Removed

The camshaft rides on high quality double ball bearings which is very nice.  Lubricating oil is supplied to the top of the cylinder head through a passage around one of the engine studs.  Oil then drains back into the lower crankcase through the cam chain passageway at the side of the cylinder.  The quality of the GY6 engine components looks to be very good.  The piston and rings also look like decent pieces.  The connecting rod appears to be another high strength forged part.  Long studs extend out from the engine case and the cylinder and cylinder head assembly are torqued down by 4 nuts.  Just beware that these nuts should not be over-torqued.  Some people have done this and stretched these studs before.  Fortunately, the studs can be replaced, but better to just be careful in the first place and no over-tighten the head nuts.   

GY6 Piston and Rod

Piston and Rod of Honda GY6 Engine

The Honda GY6 engine crankshaft is supported by big ball bearings (very nice).  This engine can spin up to 9000+ RPM!  This is a testimony in itself to the good design of this engine.  The Honda GY6 engine design has an oil pump and pressure lube system that supplies the lubrication needed to help the engine survive at such high RPMs.  In the picture below, with the flywheel side engine case removed, you can see a small sprocket at the bottom right.  This small sprocket is chain driven off the crankshaft and it appears to spin the oil pump.  In the upper left hand corner is the gear set up for the electric engine starter. 

Engine Side Cover Removed

Side Case of GY6 Engine Removed

Overall, the Honda based GY6 engine in the Twister Hammerhead is impressive.  Considering its older design and technology, it is a relatively powerful little 150cc engine.  Overall, the GY6 engine design appears to be very good, and with proper maintenance (especially keeping the air filter clean) it should last a long time.  This little Twister Hammerhead engine does work very hard, and that makes it all the more impressive when you consider how small the 150cc displacement really is!  Frequent oil changes are also in order to help ensure a long life from this hard working engine.  The Twister Hammerhead engine is able to move this go kart around quite well especially considering its small size.  As always, there are those that want more power.  Horsepower and performance can be addictive.  At first, a certain performance level might seem adequate and be enjoyable.  After becoming accustomed to this level of performance and building up a tolerance of sorts, then it's possible that this power will no longer satisfy.  Herein often begins the pursuit of more power.  Some people believe that you can never have too much power.  To give you an example of where this can all lead, below is a video showing a Twister Hammerhead with a transplanted 100 HP snowmobile engine!





REPLACEMENT ENGINE - After years of running the engine hard in our Twister Hammerhead buggy, it finally failed beyond the point of being worth repairing.  I had modified the engine in the attempt to squeeze out more power, and it was constantly run at high RPM under heavy loads.  We were driving at the dunes when the engine finally died.  When I tried to re-start and get it moving again, it just didn't want to keep running.  At first, I thought that the clutch failed again like it had in the past.  Once before, the clutch had failed such that it was engaged 100% of the time.  When trying to crank the engine, the starter motor would actually move the whole buggy because the centrifugal clutch was stuck in the engaged position.  As a result, the engine could not spin over freely when cranking and it just didn't want to start easily.  It would be like trying to start a car with a manual transmission while it was still in gear.  When you turn the key, the engine turns slowly and the car will move a little.  Anyway, the clutch failed on this buggy before at the dunes and caused similar symptoms as I was experiencing this time.  When I eventually had time to tear into the engine, I pulled the side CVT cover off expecting to find a failed clutch again.  I was surprised to discover that the clutch looked just fine.  Well, then what was causing the engine to crank slowly and not want to keep running?  I tried to fire up the engine and it would sputter and run slowly, but when I tried to rev it up it would stop.  The engine cranked slowly and seemed to be "dragging" somehow.  After spending a little more time, I decided that the engine was acting like it was partially seizing.  It might have been damage to the cylinder wall or piston and/or rings.  It might be a crankshaft bearing going bad.  I wasn't sure, but I knew that the engine would not turn over freely.  At that point, I decided that it was not worth tearing deeper into the engine, since it was already a well used (and abused) engine.  I decided to look for a suitable replacement Twister Hammerhead engine.  I'll share a little of my experiences here in case you are considering to do the same on your buggy. 

First of all, I decided against buying a used Twister Hammerhead engine on eBay because there was no way to know for sure what I would get.  Plus, these used engines are not easy to find anyway.  Even if I could find one on eBay, the engine might be as bad as the one that I already had.  So, I did some research and determined that the Twister Hammerhead used a SHORT CASE GY6 engine.  That's an important detail to emphasize.  The Twister Hammerhead needs a SHORT CASE GY6 engine.  If you get a long case 150cc GY6 engine, then it will NOT work.  In any case, I found a new short case GY6 engine complete with carb, CDI, ignition coil, and voltage regulator for only $299 + $60 shipping on eBay.  I didn't really need the extra parts like CDI and ignition coil, because I planned to use the old parts that were already on the buggy, but I figured it would be good to get the extra parts for spares.  I did end up using the new carburetor since I figured it would be best to start out with a fresh carb on the engine.  My old carburetor was modified and re-jetted for the original engine which I also modified.  Since this new engine was bone stock, then I figured a stock carb would be better anyway. 

I ignorantly expected that the new engine that I bought would be 100% plug and play.  It was not.  One of the biggest differences that I discovered was the rear of the engine case where the Twister Hammerhead engine mount attached was NOT the same.  Simplest way to describe it is that the original Twister Hammerhead might have started off as a standard short case GY6 engine, but it was modified in the back to allow a different way of mounting the engine.  The standard GY6 engine used in scooters has a rear drum brake and the wheel attaches directly to the engine output shaft.  Unlike the scooter, when used in the Twister Hammerhead go kart, the rear output shaft has a sprocket and the reverse gearbox attached.  As a result, the factory did some machining on the rear case to remove some bosses and features used with the drum brake on the scooter.  Then they drilled a couple extra holes and attached a reinforcing bracket.  This machining also provided some more clearance for the sprocket on the go kart.  The bracket provided some extra strength where the engine hung at the rear engine mount.  Below you can see a picture of the original Twister engine and what I'm describing here. 

Hammerhead GY6 Engine   

Original Twister Hammerhead Engine with Factory Modifications

Since the short case GY6 engine that I bought did not have these modifications, I needed to figure out what to do.  It was easy enough to cut off the brake pivot post with a hacksaw, but machining the other part of the engine case was not something I was able to do.  I do not have access to a milling machine to be able to machine the rear engine case.  I thought about using a die grinder with a carbide cutter to remove the boss by hand and then drill for the reinforcement bracket.  I didn't think about this too long, because it would be very difficult to get the surface flat trying to do this by hand.  In the end, I decided to just leave the engine case alone and make a new engine mounting bracket that would fit the wider boss on the new GY6 engine.  I used a long bolt that I had and took some steel flat stock and bent a new bracket using a vise, big hammer, and a fair amount of time to form the right U-shape.  I drilled a hole and put the long bolt through and tack welded it in place.  Going this route, some other complications were created.  With the wider boss and the wider bracket, now there was interference with the chain.  I was able to fix this by shimming the sprocket to the right a little more.  I did this just enough to make a small amount of clearance between the side of the chain and the bolt head that goes through the bracket.  You have to be careful not to shim it too much, because that affects the amount of spline engagement with the engine shaft that goes into the reverse gearbox.  In addition, the drive sprocket still needs to be lined up with the larger sprocket on the axle.  That's why I shimmed it as little as possible.  In my case, the sprockets were still lined up pretty well.  As it turns out, before the extra shimming, the drive sprocket was a little to the left anyway, so when I shimmed the sprocket  to the right to make more clearance for the chain, it wasn't too far off in alignment with the lower sprocket.  The sprocket alignment is important because otherwise the chain will be put under additional strain and wear if there is too much misalignment.  In the picture below, you can see the new engine mount bracket and also where I put the additional shims for the sprocket.  I should also note that after shimming, the reverse gearbox was moved to the right a little, so I needed to use some washers to shim where the front of the reverse gearbox bracket bolts to the engine case. 

Twister Engine Mounting

Replacement GY6 Engine in Twister Hammerhead

I noticed another difference with this replacement engine.  The end of the engine output shaft was not cross-drilled for a cotter pin.  This was a big one because the nut on the end of the shaft was not supposed to be screwed on tight.  It was supposed to be slightly loose and backed off a little so that the gearbox and sprocket would not be tightly bound together.  That's why the original Twister Hammerhead engine came with the end of the shaft cross-drilled, so that a cotter pin could be used to secure the castle nut and prevent the nut from loosening all the way and falling off.  At first, I thought that I could just drill the shaft.  I started to drill the end of the threaded engine shaft, but I could tell that the shaft was too hard and it was not going to work easily to try to drill all the way through it.  I thought about using 2 nuts and locking them together, but there was not enough length of threaded shaft to be able to fit 2 nuts.  Finally, I had a simple idea.  I took the castle nut and carefully used a vice to compress and slightly bend the tabs that make up the crown of the castle nut inward.  This created an interference fit so that the nut would not thread on easily.  In addition to this, I used some permanent loctite (red type) on the threads.  The combination of the homemade "locknut" and the Loctite have been enough to keep the nut securely on the end of the shaft so far.  There were some other small differences between this replacement engine and the original Twister Hammerhead engine.  The kickstart shaft that comes out of the left side of the engine case was in a slightly different position.  On a scooter, this small difference would not make much difference, but on the go kart which uses this shaft as part of the muffler mount, then it required a little work to get the muffler to mount up properly.  Another difference between the engines was that the new engine did not have the fittings on the engine case in order to attach the oil cooler.  That was one of the things that I really like about my Twister Hammerhead kart.  It had the remote oil cooler with a spin on oil filter.  That was a nice addition, but there was no way to use this on the new engine, because the engine case was not machined into the oil system with provisions for hooking up the external oil cooler.  As a result, it will be especially important to use good quality oil in this engine.  After break in with conventional oil, then I plan to switch over to a good quality synthetic oil.  Synthetic oil can better handle the higher heat that this small, hard working air cooled engine is subjected to (especially during hot summer days).  Well, that's about all that I can recall on the differences between the two engines.  If you are looking for a replacement Twister Hammerhead engine, then a short case GY6 engine can work as long as you are aware of some of these differences and the modifications needed to make it work.  Otherwise, some people transplant an entirely different engine into their buggy, but that requires even more work and more mechanical know-how to accomplish.

 

 

 

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