# Longboy's "BRATZ" Model Gas Engine.



## Longboy (Jul 18, 2017)

PREMIERING SOON!


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## Longboy (Jul 21, 2017)

Twin cylinders, four cylinders....gotta be something in between. Coming back again from their forgettable intro in the automotive world in the 80-90's is the 3 cylinder engine. From Ford Fiesta and Mini Cooper as the latest  offerings to Yamaha's biggest seller in large motorcycles, 3 cylinders are making headway in efficient, high output applications in small cars cutting the cost of driving. In the case of the BRATZ engine, it's a look back at historical means of power by 3 cyl. engines found in industrial and agricultural usage. After the single cylinder and excluding aviation radials, it is uncommon to find odd number cyl. engines being modeled.....let me show you my version!


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## Longboy (Jul 22, 2017)

BRATZ is the same design with some valvetrain changes as the BEST FRIEND engine. The crank is a heavier unit using .250in. throw pins on 1.75in. crank webs.




4 main bearings. Center brass  and end Oilite bushings. Aluminum  Parksy design cradle in black satin. Unit weighs just under 4 lbs.


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## Longboy (Jul 23, 2017)

A 6 in. long block receives some 1.125 in. bore DOM liners  at a 2 in. bore spacing.





The block is relieved for the cam drive sprocket and the block hugger camshaft along its length to clear the lobes.


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## kadora (Jul 24, 2017)

Hello Longboy
Could you describe how did you machine crankshaft ?
Thanks


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## Longboy (Jul 24, 2017)

The crankshaft is a pieced together unit. The appropriate length of brass round stock is center drilled 5/16 in. for the pins that ride on bearings. The offset holes for throw pins are drilled 1/4 in.  The round stock is then parted off for all the webs you need and referenced marked for sequence and direction. On BRATZ the webs are half inch tall and the pair of center bearings are 3/8 in. A 1 3/8 in. ( plus several thousanths for rod and bearing clearance) long pin of 5/16 in dia. is knurled each end and pressed onto both webs with some locktite. Two center bearings, two pairs of crank webs here. There is a pair of thin SS washers between webs and the brass bearings, one each side of bearing.
   The throw pins are similar length as the con rod is 3/8 in. wide stock.  the pins are locked to web with 10-32 set screws w/ locktite on screw.  A flat is on throw pins for the cup of the set screw.
   This method works but is difficult and need patients and diligences to get the unit rotating without axial and radial run out ruining your day as you tighten down each segment. This is one reason on this engine I up sized the throw pins and height of the webs from the BEST FRIEND engine....to gain more surface area, pins to webs. :thumbup:


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## Longboy (Jul 24, 2017)

This is the lifter bar. Exhaust valves only. Brass guides are set screwed in. Bar screwed above camshaft into block.




The lifters themselves are rivets found in the fastener bins over at ACE Hardware so I don't even bother making my own. The mushroom  will be turned down to a  smaller diameter to clear cam lobe set screws. A flat is milled on the mushroom dome to ride the cam lobe and the stem is drilled down into to take a piano wire push rod.


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## Longboy (Jul 25, 2017)

A 3/16 in. shaft with cam lobes of 3/8 in. CR. The drive is by MXL sprockets. Brass bearing on shaft end and a roller bearing in sprocket guard.




(Thats a funny looking brass lobe on there Longboy! ) ..... I'll get to that next post.


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## Longboy (Jul 26, 2017)

A distributor at a right angle to camshaft is a change up from previous engines. Compact and close to block, its distributor cap is a 3/4 in. internal dia. PVC end cap.


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## Longboy (Jul 27, 2017)

The camshaft and lifter bar mounted to block.




Distributor just clears the lifter block.




The bevel gears are 24T at .5 modulus. Just the right scale! Knurled screws retain the distributor cap.:thumbup:


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## Longboy (Jul 28, 2017)

An intake valve block and manifold.




Delrin manifold has Traxxas carb on a pipe fitting angled out just enough for carb body to clear. 





O-rings seal the two piece valve block.


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## Longboy (Jul 30, 2017)

The head fining a tedious operation. By pattern to get around various holes and execution.  Eighteen rows @ two or three passes to get the depth. With shallow combustion chamber and a one inch height aluminum stock, the brass valve guides are near flush with the top of head





On the exhaust side, a pair of ports are symmetrical for #2 & 3. #1 had different ideas and went in another direction.....the black sheep of conformity.





Plenty of room in this chamber with a 1.25 bore. 10mm threads for CM-6 spark plugs and single ex. valve.  The relief into the spark plug thread is for the in. port as the ports were above depth of combustion chamber. Tiered circles around chamber would be inner for cylinder liner seat and the outer tapered for the O-ring seal.


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## Longboy (Aug 1, 2017)

The piston and rod assembly lightened on the con rod, they weigh 1.34 oz.




The O rings on the pistons are Metric riding on an inch sized bore. Close but slightly smaller outside diameter....I hope they don't give me grief losing compression in running.


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## Longboy (Aug 3, 2017)

The rocker arms and their pedestals are all brass with SS hardware. Easier to cut and shape than steel arms in this scale.




With half of the valve spring below the head surface the rocker assembles show a low profile compact fit as do the spark plugs to the cylinder head. There are no lock nuts for the push rods. Drilled for a 6-32 threaded hole, the 3/32 in. dia. piano wire push rods slip up in the arms and a set screw within then maintains the valve clearance.  Some blue Loctite on set screw holds its position.


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## Longboy (Aug 4, 2017)

Using heat sink material on the Best Friend engine worked so well I bought some more with greater fining area and bolted to the perimeter of the cyl head.




Also a couple bars on the upper engine block, BRATZ has little chance of becoming a hot head in the Arizona summer.


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## Longboy (Aug 5, 2017)

Almost finished here, the top side of engine.





   Block and head with all external components except wiring.




   The legs on the crank cradle. The three slots allow pass through of rod caps to separate rods from crank shaft while in the cradle.




   Loading of the bores joining crank to block.


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## Longboy (Aug 5, 2017)

The "BRATZ" engine is now completed!woohoo1




   Work was started first week of April.




   Finished early July and up and running well about 10 days later.




   The largest flywheel I have used, spacers under the feet were added later to clear my hand for hand starting.




   Troubleshooting the fuel and timing to get the right sound, more displacement and same size exhaust porting as the BEST FRIEND engine gives a sharp Ba-Brat cadence to the exhaust pipes, hense its namesake.




1st. summer build of an engine...........I'm outta my mind and couldn't help myself. :shrug:




But it gets done!:thumbup:

Video premier coming next post.


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## minh-thanh (Aug 5, 2017)

very very nice !!


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## Longboy (Aug 6, 2017)

Welcome to the "proof of life" video of the BRATZ engine. Enjoy:thumbup:




[ame]https://www.youtube.com/watch?v=vs7Yh_FAWi8[/ame]


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## Cogsy (Aug 7, 2017)

Sounds very healthy indeed!


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