How To Un-lock A Seized 400 ...??

I just took delivery of a very clean but locked up 73' CR400 engine ...

Seems water has migrated through the carb into the bore just to add to the problem.

The piston is at the bottom of its stroke.

I have left WD40 in the carb intake with the engine standing on end in the hope it will penetrate into the bore. I had also left WD soaking on top of the piston...

I dont know if the motor is locked in the bore or the bottom and.

I have the head off and I tried gentle through to harder hits with a heavy rubber mallet to all four sides of the barrel ... nothing moved...

Any suggestions?? - heat maybe??....

I always try to turn it from the primary side not the ignition side, too easy to break the crank threads off.

Insted of wd40, try 50/50 ATF oil and aceton (great penetreater)

Since it will all have to be rebuilt anyway. I would take a chisel to the piston and break it up.
Be careful beating or prying on the cylinder, I'm sure you don't want any bent or broken fins and or damage the barrel.

yep acetone and ATF....50/50. I posted a review on that mix..showing torque value comparisons

here ya go....

Machinist's actually tested penetrates for break out torque on rusted nuts. Significant results!

They arranged a subjective test of all the popular penetrates with the control being the torque required to remove the nut from a "scientifically rusted" environment.

*Penetrating oil ..... Average load*
None ..................... 516 pounds
WD-40 .................. 238 pounds
PB Blaster ............. 214 pounds
Liquid Wrench ..... 127 pounds
Kano Kroil ............ 106 pounds
ATF-Acetone mix....53 pounds

The ATF-Acetone mix was a "home brew" mix of 50 - 50 automatic transmission fluid and acetone. Note the "home brew" was better than any commercial product in this one particular test. Our local machinist group mixed up a batch and we all now use it with equally good results. Note also that "Liquid Wrench" is about as good as "Kroil" for about 20%of the price.





Excellent info. In fact, old ATF saved in a bucket is a great rust remover by itself

I used Deep Creep on an siezed IT175 recently. Worked a charm!

Make sure you start soaking the cylinder studs too.

Thank you for all the suggestions guys (and gals )

I finally had a go at getting it moving today ... but had no luck! ... I only had a short amount of time to play with it. I did make a puller though (or pusher!) that I think may get it moving after I give it a go in the magic ATF/Acertone mixture that has been suggested.

The problem with the piston at the bottom of the stroke is that; maybe its not the piston that is stuck but rather the bottom end. Not sure if there was oil?...may want to soak (FILL gear box) the bottom end in ATF-Acetone too?...
I hate to see the crank damaged

Another fantastic rust mover is a mix 50/50 ATF and Diesel. I have used this mix in a spray bottle for rusted nuts,bolts ect, and have had to use this mix to unfreeze engines as well. Give it a shot, Diesel is a fantastic rust eater, and the ATF gives good libe to aid in removal.

I got apart a 1982 Yamaha Maxim 750 that was left outside for over 3 years. Out of 4 cylinders, 3 pistons were rusted in place. I filled all cylinders with Marvel Mystery Oil and worked the rear wheel with transmission in second gear, I kept feeding Marvel for about a month before trying to remove the topend. The head came off fine and the pistons pushed out with a large nylon rod against the piston and hitting the rod with a 2 pound hammer. I was able to save all 4 pistons and all 4 cylinders cleaned with a honing

I like your pusher idea. The problem is though that if the piston is at bottom dead centre you wont push it anywhere by pulling against the studs like that. The studs are screwed into the crankcases and you are pulling against them and the crank. If the piston is partway down it might work but you are still attempting to turn the crank at the same time.

if have tried to turn the cranck whit removed head, it seems to be the cranck that is blocked by rust, probebly the piston in the cilinder ass well. i have yused an elektric contact cleaner spray to penetrate the oxidised parts, and it works very well.
goodluck, cheers