661_Minis
New Member
Aug 13, 2021
#1
#1
Hello, I’m building a predator 212 non hemi and need advice on achieving a 10.5 to 1 compression ratio, how much should I shave the head? I haven’t measured how deep the piston is in the hole yet because I already took the whole engine apart and I’m waiting on my ring compressor and other parts too come.
HERE’S WHAT I ORDERED
3.308 rod
Billet flywheel
292 nr racing cam
26lbs springs
1.1 champ rockers
.18” mls head gasket
3 stage center exist header pipe
Chinese vm22 carb & manifold (with jet kit)
I will be ordering CM pushrods after I measure and see what size length I need. Also I am planning on porting the head, I just need more tips and advice. Give me your guy’s opinions on the build, if there’s anything I should do or change, also this engine will be going on a mini bike.Thanks
SAS289
Well-Known Member
Aug 13, 2021
#2
#2
For any compression estimate you should assemble the crankshaft and rod with the piston and measure how far in the hole the piston is at TDC with the side cover on.
Flat top or dished piston?
I wouldn't know what you would need but these are the things others need to answer your question. I only have a compression chart for 196. 212 may be slightly different.
Why did you get a standard length rod if you wanted higher compression?
Here's the 196cc chart that tells you what you need to know to get the compression you want.
Harquebus
Well-Known Member
Aug 13, 2021
#3
#3
SAS289 said:
...Why did you get a standard length rod if you wanted higher compression?
Bam! Should've asked questions before buying. 
P
panchothedog
Well-Known Member
Aug 13, 2021
#4
#4
Hey 661 I have built a couple of pretty hot predators this last year. Both were non hemi's the exact same engine you have. You are on the right path. To get more compression mill the head .040" BUT ALSO install a flat top piston. The stock piston is dished to the tune of 3cc. About the same as milling another .020" off the head. Don't know where you get your parts from but I have purchased them from Go Power Sports and OldMiniBikes warehouse for about $20. Also with the stock length rod you can use a stock .010" head gasket . I don't know what the compression ratio works out to be but it is about all the block can handle. I strongly recommend you use the head stud kit instead of the stock head bolts. You can get a lot more clamping force w/o worrying about stripping the threads on the block.
I torque mine to 28 lbs and recheck after the first 2-3 heat cycles. Don't feel bad about not getting the longer connecting rod,I put one in my first engine and it puts the piston flush with the deck only to be told by the engine builders at both formentioned shops that the crankshaft flex in these engines at high RPM is enough that you need .030" between the top of the piston an the head. So by the time you build up gaskets to get the clearance needed it negates what is gained with the longer rod.
661_Minis
New Member
Aug 14, 2021
#5
#5
panchothedog said:
Hey 661 I have built a couple of pretty hot predators this last year. Both were non hemi's the exact same engine you have. You are on the right path. To get more compression mill the head .040" BUT ALSO install a flat top piston. The stock piston is dished to the tune of 3cc. About the same as milling another .020" off the head. Don't know where you get your parts from but I have purchased them from Go Power Sports and OldMiniBikes warehouse for about $20. Also with the stock length rod you can use a stock .010" head gasket . I don't know what the compression ratio works out to be but it is about all the block can handle. I strongly recommend you use the head stud kit instead of the stock head bolts. You can get a lot more clamping force w/o worrying about stripping the threads on the block.
I torque mine to 28 lbs and recheck after the first 2-3 heat cycles. Don't feel bad about not getting the longer connecting rod,I put one in my first engine and it puts the piston flush with the deck only to be told by the engine builders at both formentioned shops that the crankshaft flex in these engines at high RPM is enough that you need .030" between the top of the piston an the head. So by the time you build up gaskets to get the clearance needed it negates what is gained with the longer rod.
661_Minis
New Member
Aug 14, 2021
#6
#6
Okay thanks for the info, I’ll let you know in a few weeks how the engine build turns out. Thanks
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