Broke two heads in two runs??? Why??

the piston to deck hight looks stock and its far enough below level it should never hit the head.

your problem is over tightning your head bolts ! put them on too tight or tighten one all the way and not torqueing them in the proper pattern can and will cause the head to warp andwhen heated it will expand and crack

the knocking sound seems like a tottaly seperate issue... try turnning the motor over quickly with the head off and the jug torqued on with spacers by pedaling the bike with the clutch out with your hand wile the bike is on a stand of sorts and wile you do this listen carfully for the sound... if you hear knocking with the head offf then you have a major problem and i would pull the jug and check the piston pin needle bearing... it may be in pieces ! or you may have a bad case beaing that causes a woble enough to cause a hard knock

Let me get this straight...
You baked your entire assembled engine in the oven at 500 degrees?
Wow. That's a new one.
Yes. That kind of temps will damage stuff in an assembled engine!
I don't even know where to begin telling you how bad an idea THAT was.

I dont know what sort of paint your using, but even VHT engine enamel paint doesn't need that amount of heat to cure.

180°C = 356°F

http://www.speco.com.au/vht_faq.html

How do I apply VHT Engine Enamel?

A finish coat is only as good as the surface it is coating. We are frequently asked if VHT Engine Enamels can be painted over a primer or other old paint on the engine. To obtain the best results we recommend the parts be stripped clean of any old paint or primer. Wipe down the surface to be painted with grease and wax remover and prep with either Shellite or brake cleaner. For maximum adhesion and colour consistency we recommend applying VHT Engine Primer (SP148).

If the old paint is intact, remove any dirt, rust, grease, loose paint, etc and lightly sand to a dull finish. Wipe down the surface with grease and wax remover then prep with either Shellite or brake cleaner.

The topcoat is only as good as the base coat or condition of the metal. If the substrate gives way, the topcoat will follow.

Carefully follow the instructions on the back of the VHT can.

Paint the Surface: Do not use in temperatures of less than 21°C on parts that are less than room temperature. Apply VHT engine coatings directly to the surface to be painted. Apply a smooth even coat and lightly sand for a primer coat or spot prime. Hold can 12 to 15 inches away from surface and spray in one direction using long even strokes. Repeat the process in the opposite direction. Wait approximately 30 minutes before recoating. Excessive coating build-up is not necessary or recommended and will cause blistering and loss of adhesion. Apply only enough coating to color surface evenly. If touch-up or recoating is necessary after 4 hours, a light sanding may be needed.

Curing: VHT Engine Coatings will air dry in one hour and cure overnight. Baking for 15 minutes at 150°C and then 30 minutes at 180°C improves finish and provides a more durable surface for handling, resistance to salt-spray, chemicals and humidity. Curing may be accomplished by the inherent heat of operation, as encountered in engines, exhaust systems or by baking in an oven, infra-red, blow torch, etc. Starting the engine and running at idle for 15 minutes and letting cool can also cure VHT Engine Coatings. Restart the engine and run for no more than one hour and again let cool. If the engine is operating normally, this will bake on the finish. All curing must be done slowly.

johnsoma said:

Hi guys,
I ran a new engine twice and broke two heads in less than 15 mins. It was a prebuilt motor from ebay. Only thing I did was paint the motor with black engine paint and cook it in the oven to set the paint.
Any chance the cooking at 500degrees hurt something inside??

Mike

Click to expand...

Well, all the clutch grease is baked, all the fiber gaskets aren't happy, and I'm surprised the magneto solder connections were still good enough for it to even start.

Most of my 2-stroke engines get high temp motor paint (I like PlastKote) but I just do the head and all 3 covers not the block or cylinder.
I give them a couple coats outside and let the 100+F AZ sun bake it on all day and then cure overnight.

You can duplicate this with a Sun Lamp.
TIP:
A simple spray bottle with plain water is handy to spray off hot painted parts so you can handle them.

Do you have a torque wrench and know how to use it?
66cc book says 15# but I do mine to 20# cold 5# at a time in a star pattern, then tweak back to 20# again when it's hot.

I'd say just get another engine.
GasBike has un-assembled 66cc engines for $99.
http://www.gasbike.net/66cc-racing-engine-build-4028-prd1.html

You can paint individual parts until your hearts content before assembling the thing.
Good luck with it.

wait you baked the WHOLE MOTOR ANDD NOT JUTS THE BARE PAINTED PARTS !!!!

sorry your motor is junk now .......... my advice is to take it appart and order a new motor and wile waiting for the new motor ..... port your jug and ramp and cut the piston and expand the squish band on the stock head and then deck it down for higher comp...


hey man next time srsly just paint and go LOL the motor gets up to 250 pluss deg wile running so it cures any paint all on its own wile you ride !!!

i use VHT high heat pait and then i just go for a long ride and wile riding i can smell it curing , when its done curing you wont smell it any more