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230 piston in a 150 - claims to rip

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Just talked to guy who is running a 230 piston in his 150. Not sleaved and required no case work. Claims the thing rips. Anyone have any experience with this?

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That's funny you ask, out of all the times I've been through this forum. I don't think anyone has asked about it. I wish I knew the answer. It may require some timing changes along with jetting.

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I have not really looked into it... but the aftermarket 170BB kits use a 66mm piston... the same bore as the stock 230... so sounds like a bore job and a 230 piston is a cheap bb kit... I am not sure about actual compression or valve pocket location type of stuff... if he did it then I am sure it can be done... you might want to ask him if he changed anything else but the bore... also see if he cc'd the head to see what the actual compression is... I would be curious 😢

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The 230 piston will fit into the 150 with a slight over-bore, and increase its displacement to 166.7cc, an additional 10cc over stock. The compression will be very low however, as the 230 utilizes a flat top piston, while the 150 has a small pop-up. Also the compression distance is also @.030" lower with the 230 piston, meaning the 230 piston in the 150 would not come up to the top of the cylinder at TDC. I have done actual compression calculations and the stock 150 is actually down from its advertised 9.0:1 ratio (8.4-8.5:1 in reality) The 230 is right at 9:0.1. A stock 230 piston in a 150 would run, but not so good as the "Rumor" you heard. The compression would be very low. My guess is around 7.2:1 range. My opinion and expeience is to tell anyone thinking about this mod to "fuhget about it" !! If you are interested in a "real" big bore for your 150 with an actual 9.5:1 compression let me know. I have a kit that will make your 150 into a full 188cc bullet! 😢

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I figured you would chime in coeshow... Nice to know someone has all the data this guy is looking for... I am still interested in you setup... just working out all of the details for my complete bike 😢

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I just finished a modified 230, BBR frame kitted CR-85. I built the engine almost 10 months ago before a lot of this stuff was out yet. I could not get a higher compression piston for the 230 at the time, but the BBR, bigbore piston was available for the 150. It was the exact same sixe as standard 230 bore and 10:5 to 1 CR, so I used it. I cannot remember w/out looking back at my notes, but there was a .030 difference in the deck heights. I build vintage 4-stroke performance trials engines for competition and building trials engines is a totally backwards mindset in what you are trying to do w/normal dirtbikes. The world is full of engine builders that can make a bike rip at WOT, but off idle, necksnapping is hard to come by. Trials engine need as low of compression as possible to run at, and below idle w/strong power. I have never been able to get the compression ratio as low as I would like, it is very hard. Sure, you can drop the piston down, lower and lower, to attempt a low CR, but that causes major combustion issues. To get the most efficient, clean burn, the squish band height should be as close to .030 as possible. W/that in mind, a low CR is hard to come by. Another thing to remember is, ALWAYS, when displacemnt/bore size, increases, the compression ratio automatically increases. Always mock your topends up and use clay to measure your squishband gap and you will find a lot of hidden power versus an imprpoper/too wide gap. Too close and you will bend a valve, if running a domed piston. I run ATC/dished top pistons in my TL engines that are 65.5mm and have even welded the bottom side of heads to make as large of a combustion chamber as possible and still wind up w/8:1 simply because the bore size has increased. Something else that I do that will solve your issues w/the difference of .030 in the deck height. I machine the bases on 200 cylinders so they will work w/the 125 rod stroke. You can do the same w/the CRF cylinder. Simply load it into a lathe w/the jaws inside the bore and open the lathe jaws, rather than close them until they get tight enough to hold the cylinder. Then use a dial indicator to make sure you have an even plane/cylinder base. Then zero your lathe and take off the needed amont from the bottom. .030 won't make an issue out of your cam chain adjustment and you can run the proper deck height/squish band clearance. You can always run an extra base gasket if you take of a tad too much and always remember to measure and consider your head gasket thickness. It is where I made up the difference when I installed the 150 piston in my 230. The 150 and the 230 have different thickness head gaskets. If you run the 230 piston, you can't run the 150 head gasket. That might be a problem, otherwise, it should allow you to run the 230 piston if deck height is the only issue. Remember, I went the other way w/mine. I am not fond of overboring liners to the point of being too thin just to get a bigger piston in it. It is OK on a low reving bike like for trials, but it can grenade on you quick when racing MX and hare scrambles at high RPM. I will be glad to do any machining of cylinders, but anyone that is good w/a lathe will know how to do this if you will tell them the trick. Mi dos centavos, Clint

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I'm certain that the cam timing on your trials engines will make up the difference in what you consider "low" compression. The "calculated" compression ratio is a mathematical figure. Valve overlap affects running compression. Most modified 4 strokes have higher compression pistons to try and offset the usual increase in valve overlap with the accompaning "HI performance cam".(lost compression at low RPM) Your four stroke trials engine probably has 0 or next to 0 overlap, while most other four strokes will have more and more, depending on how high of RPM's will be run. I was at one time also an avid trials rider! I had a Honda TL 125, and later a TY250, and 350 Yamaha. Never owned any thing newer than 1985. The CRF230 has a .040" head gasket versus the 150's .010", and I likewise found the .030" deck height difference. The "dome" on the 150's piston is what cannot be made up for even with 2mm extra bore offered by the 230's flat top piston. Even if you machined off the bottom of the cylinder as in your post to get the proper deck height, a STOCK 230 piston in a 150 will still be a lower compression engine even if all of other proper standard parts are used .

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Sorry. I am not, and I have not had the pleasure of meeting him yet.

Mike

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Even if you machined off the bottom of the cylinder as in your post to get the proper deck height, a STOCK 230 piston in a 150 will still be a lower compression engine even if all of other proper standard parts are used .

*****This would be true, if you used a STOCK 230 piston, but I didn't. If you read my post, I used an aftermarket bigbore piston from BBR that had the same size as the stock bore on the 230. Since it was for a 150, it did have the dome. It really woke the engine up. Clint

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Your post was well written. The original post(I believe) was using a STOCK 230 piston in a 150. Even though it is doable,I still advise against this procedure.

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