Forged higher compression piston - Honda CBR 300 Forum
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post #1 of 16 Old 12-08-2018, 03:03 PM Thread Starter
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Forged higher compression piston

Is there a higher than stock compression ratio forged piston , say 13CR, available for the CBR300r?
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post #2 of 16 Old 01-02-2019, 02:59 PM Thread Starter
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I appreciate the reasoning in the forum threads stating the cheapest, most reliable bet for more power is to buy a more powerful larger displacement bike. However, some of us would seek to lighten and make more powerful whatever bike we bought, especially when we really like a particular bike. I've found a number of shops advertising online offering complete bolt on 305cc kits, cylinders and all for the CBR250r. I calculate one of those kits would give our CBR300r 348cc. The kits from some sellers are optionally available with ported heads and higher than 10.7CR pistons. My question: when Honda stroked the crank of the 250cc to 287cc for our CBR300r did they include a beefier crank, rod, pin and bearings? Am I dreaming to think that with a fuel controller, high flow air filter, no cat. no sensor full exhaust with a short muffler, 348cc, 13CR piston, and ported heads our little light weights should have approx. 45hp at the wheel?
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post #3 of 16 Old 01-02-2019, 10:40 PM
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i believe if u go on u-tube that dabinche has tried all u need with sometimes dubious results (but he has a lot of good info). he's on here also but i can't remember as who.
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post #4 of 16 Old 01-02-2019, 11:35 PM
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I appreciate the reasoning in the forum threads stating the cheapest, most reliable bet for more power is to buy a more powerful larger displacement bike. However, some of us would seek to lighten and make more powerful whatever bike we bought, especially when we really like a particular bike. I've found a number of shops advertising online offering complete bolt on 305cc kits, cylinders and all for the CBR250r. I calculate one of those kits would give our CBR300r 348cc. The kits from some sellers are optionally available with ported heads and higher than 10.7CR pistons. My question: when Honda stroked the crank of the 250cc to 287cc for our CBR300r did they include a beefier crank, rod, pin and bearings? Am I dreaming to think that with a fuel controller, high flow air filter, no cat. no sensor full exhaust with a short muffler, 348cc, 13CR piston, and ported heads our little light weights should have approx. 45hp at the wheel?
I'm going to say this with a BIG IF--If you could get anywhere near 45hp out of this bike no matter what you did to it- it would be short lived and unreliable for street use--the engine as a whole was not designed for that big of a change--add a quality pipe and fuel controller maybe drop the counter one tooth and that's about it

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post #5 of 16 Old 01-03-2019, 12:14 AM Thread Starter
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Thanks 9th Honda & motorboy. I watched dabinche's videos re his big bore build failure, very discouraging. I did note the 250r piston is a little taller than the 300r's, nets a 13cr when put in a 300r.
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post #6 of 16 Old 01-03-2019, 02:28 AM Thread Starter
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Thanks 9th Honda & motorboy. I watched dabinche's videos re his big bore build failure, very discouraging. I did note the 250r piston is a little taller than the 300r's, nets a 13cr when put in a 300r.
It occurs to me that dabinche weighed the B & B big bore piston and found it weighed several grams more than the stock 286cc piston, and that the stock 250cc piston, being taller than the same bore size 286cc piston would also outweigh it. No mention is made of rebalancing the engine after either of dabinche's mods that ended with blown engines. It seems reasonable to think that a big bore kit with a piston with rings and pin exactly matching the weight of the stock 286cc components would solve the big bore unreliability problem. Some big bore suppliers offer newly cast cylinder assemblies which allow stock thickness sleeves and cylinder walls rather than bored and resleeved stock assemblies like the B&B kit dabinche used, which should even further increase the reliability of the mod.
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post #7 of 16 Old 01-03-2019, 03:24 AM
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Just throw away the head gasket and use some gasket goo out of a tube instead.
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post #8 of 16 Old 01-03-2019, 03:47 AM
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Just throw away the head gasket and use some gasket goo out of a tube instead.
It certainly is the kind of hypothetical scenario which makes no sense at all in terms of performance, reliability or cost. Next up will be a nitrous kit for the bike.

Faster is not always better...!
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post #9 of 16 Old 01-03-2019, 09:31 AM
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Just throw away the head gasket and use some gasket goo out of a tube instead.
Just had a sixtys flash back- I remember some trying that-- never worked for long-
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post #10 of 16 Old 01-11-2019, 02:08 AM Thread Starter
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Quote from CBR250r bored to 300cc racer Ari Henning: "The 250 piston with the 300 crank yields approximately 12:1. Tightening the squish will help reduce detonation (by reducing pockets of fuel charge) but will also bump compression a point or two. Setting squish is part of proper tuning so its something you should do. Typically shaving 0.008-0.010 off the cylinder base or running a thinner 0.012-0.015 base gasket does the trick."
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Last edited by Sam R.; 01-11-2019 at 02:17 AM.
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