Honda CRF230F - Big-Bore Project NOT

January 22, 2004 version

 Introduction   Modifications   Big Bore   DIY Notes   Carb Notes   A look back 

FLASH - see Jan 22, near the bottom!

After doing the initial modifications during November, 2002, and into early 2003, I rode the 230F about a year. Finally, I decided that the engine needed more oomph, so I began the quest for more power. As of November, 2003, no one had yet made a big-bore kit for the 230F. Its smaller sibling, the 150F, had aftermarket parts aplenty, including cams and several big-bore kits. So, I decided to do a big-bore kit on my own.

The first step - find someone to make the piston

JE Pistons makes aftermarket pistons for a variety of engines. I talked to Brian and he told me that they could indeed make some larger pistons for the 230F. I needed to make a mold of the combustion chamber, send it and a standard piston to them, and after several weeks, voila, new pistons would arrive.

I wanted a larger bore but not one that would compromise the engine. A friend had tried making a 280cc kit, but it was unstable; the cylinder moved around. So, I decided to go for a 2 to 3 mm larger bore, depending on the liner thickness. Brian told me that a minimum liner thickness of .120 inch after boring is recommended. Also, I wanted a higher compression than the 9.0:1 stocker, perhaps 10:1 or 10.5:1. I didn't want to go to a high-compression piston to minimize the need for high-test gasoline or racing fuel.

A 2mm overbore yields a 237cc displacement; a 3mm overbore yields a 244cc displacement; 244cc sounds great!

Note: For custom orders from JE Pistons, you must order a minimum of four pistons, and the estimate is $500-$600 for the set, which includes four each: pistons, pins, rings, and clips. So, I'll have three leftover sets after this is finished. Perhaps I'll sell a couple if anyone is interested.

I might also need a new head gasket, depending on what I finally choose for bore size. I might be able to get by with trimming the stocker. So, Cometic Gaskets would possibly also be in the loop.

I needed to order a new stock Honda piston to send to JE Pistons with the combustion chamber mold. I also needed to order a new stock Honda cylinder to use for the overbore, so I could retain my stock cylinder and return to a stock setup in the event of any problems during the project. I also needed various gaskets and an o-ring. I ordered the stock piston, cylinder, and other parts from Service Honda on 11/5/03 and then waited.

Nov 12, 2003 - the cylinder, piston, and other parts arrived today. Time to check the liner and see what I can get by with on an overbore.

Oops, problems. When I inspected the cylinder to see how much overbore I could get by with, I found that the liner thickness is 2mm; the suggested minimum after boring is 2mm. So, the liner is already too thin to overbore, which means that I have to have install a new, larger liner. The old liner must be pressed out, the cylinder bored, and a new larger liner pressed in.

However, there is another problem. There is not much room in the cylinder for a larger liner. Have a look at the picture of the new cylinder (click it to see a larger version or click here for a zoomed version). Notice that there is only 1mm between the stock liner and the cylinder bolt hole (this is the only hole that is so close). This leads me to believe that the new liner can only be 1mm larger in radius, or 2mm larger diameter/bore, yielding a 237cc capacity. This is barely acceptable.

However, there may be an answer. I think the lip at the top of the liner is larger than the liner od, so it's possible that I could get a liner more than 1mm larger in there. The lip will have to be 'keyed' to the bolt hole, that is, drilled out to match the hole. I'll be talking to LA Sleeve, Millennium, and JE Pistons this week to see what can be done. I have not given up all hope yet.

Nov 17, 2003 - I talked to Frank at Engines Only/ about a 230F big-bore kit. He has a prototype kit that is running in a 230F now. He said he will run the kit for a while, then tear down the engine to see what it looks like. If he has to make any changes, he will do so and test again. Eventually he will have a kit for sale. He would not reveal any details, including displacement, price, or dates. I'm guessing he won't have anything for at least a month or more.

After reading about BBR's 240 kit in the December issue of Dirt Bike (p.24), I sent BBR an e-note. Their reply, in part:
"Right now the kit isn't something that we have released although we should have it available soon, most likely after the first of the year is when we should have them for sale."

Nov 18, 2003 - I talked to Kurt at Millennium Technologies about a new liner. He thinks that a 3mm overbore is possible. I sent the new Honda cylinder to Millennium this afternoon. They'll let me know what they can do (next week).

Nov 24, 2003 - Kurt at Millennium Technologies called. They got my cylinder and measured and cogitated and decided that they can make a 3mm overbore liner for around $220. We had a chat about options. They felt that a 4 mm overbore is not a good idea because of the proximity of one of the cylinder bolt holes (as I expected). I told them I would start the new-piston process after talking to JE Pistons. Millennium will put the cylinder work on hold until they get the pistons from JE.

I talked to John at JE Pistons about the piston kit. It turns out that rings are available only in certain sizes. The 3mm overbore yields a 68.5mm total and there are no 68.5mm rings available. So, I'm now stuck with a 2.5mm overbore yielding 68mm bore and 240.4cc displacement.

Compression ratio goes up when you overbore, but I may decide that I want even more compression and a slight dome on the piston. We discussed using the premium/slipper forging, which produces a slightly lighter piston and requires a shorter pin. This will all put less load on the bottom end, and it adds about $15 per piston to the cost.

John told me how to make the combustion chamber mold from 'bondo'. I also need to cc the current combustion chamber, so we can calculate the exact compression ratio. I need to measure the deck height, which is where the top of the piston is in relation to the top of the cylinder at TDC. Also, I need to measure the engine case opening to see if the larger liner will fit without machining.


There has been a slight hitch in the project; the project bike is not available for disassembly at this time. I'll get back on track by the end of the week, I hope.

Dec 1, 2003 - I talked to Brian at JE Pistons. We decided that the new piston should be a flat top, just like the stock Honda piston. So, I don't need to make a combustion chamber mold or do any other cylinder or head measuring. BUT, I still need to find out how large the liner opening is in the engine cases. If the opening is too small to accommodate the new larger liner, the cases will have to be machined, and this is not something I want to do. Beside the expense, the work to tear everything down and then reassemble is long and tedious.

Dec 3, 2003 - I talked to two people from different engine-performance companies. Each is the senior or only technical person working on a 230F big-bore project for their company, and neither wanted anything further disclosed about their work or names. I am able to report one single fact - to overbore, you need a new larger diameter liner installed, and that necessitates boring the engine cases. The later was the haybale that broke my back ;-)

Also, there are issues with boring the engine cases. There is very little clearance to the cylinder/head studs, and there is an oil galley in the area also. Both of these represent problems that I am unprepared to deal with; I've never done this kind of thing before, so I have no working knowledge of what you can actually get by with and still have a reliable end result.

One engineer commented that the 230F is very near the limit as it has been built by Honda. The engine was designed as a 200 some years ago, and making it a 223 took the design very close to its physical limits. Perhaps that is why it's not 230cc; Honda may have wanted to stay on the safe side. This does not bode well for the aftermarket big-bore kits.

I may go for a kit when BBR, Powroll, Engines Only/, Thumper Racing, or XRs Only offers one next year. I won't be the first to buy; I'll want to hear how reliable they are first. But my own efforts are now fini.

I'll be getting the 250X next summer, right after a 290-300 big-bore kit is released (by one of the previously mentioned companies). There doesn't seem to be such a problem with overboring the 250X engine.

Dec 13, 2003 - new developments. Check this web page; all hope is not lost.


The end result - someone else did it first

Jan 22, 2004 - great news - BBR and Engines Only announced their big-bore kits. Check their web sites for details.

The BBR kit consists of a new piston, rings, pin, and gaskets. Bore the cylinder, install the new piston and other parts, and go riding. The kit increases displacement to 233cc, although BBR calls it their 240 kit. Price is about $170. I'll be ordering mine today.

The Engines Only kit includes a resleeved cylinder (exchange), new high compression piston with rings and clips, all top end gaskets and jetting. This kit requires case machining. Engines Only also sells the lower end gasket kit and provides a complete engine installation service. Prices range from $450 for the kit only to over $850 for the complete installation. They are also offering a whole bunch of new aftermarket parts for your 230F; they plan to announce them very soon. To see what's possibly coming, go to their 250 kit page. In your browser, choose 'select all' from the menu. You'll see the new parts in the highlighted area.

Here's what I ended up doing.

Other   (Note - click the underlined names to get to their web sites.)

JE Pistons   714-898-9763
Minimum order - 4 pistons
See web site for order form

Millennium Technologies   888-779-6885 or 920-893-5595
Cylinder liner and bore to match piston
See web site for order form

Cometic Gasket   800-752-9850

Service Honda   219-932-9332
Cylinder, piston, rings, pin, base gasket, head gasket, cam cover gasket

BBR Motorsports   888-668-6227  253-631-8233
240 big-bore kit

Engines Only   408-374-4297
250 big-bore kit and other