The has bee alot of talk about 350x heads and all that good stuff in here in recent days that i started to think about doing a RX conversion, but i got alot of questions.
1.Will i get more power out of just using a whole 350x motor or using a 350x topend with a 250x/300ex bottom end?

and of course i will have the topend bored and stroked. I think thi iwll be a awesome st up for woods and CC racing. I will proably stick the motor in a 88' R frame.

If i bore and port the 350x head to like 425cc what kind of power should i expect?
And will it be faster then moddified R's?

I am pushing to use the whol 350x motor because of the 6 gears and kick start.

Also would i be able to get more power if i just use a 250x engine bored to like 400?

Thanks for anwsering if you do.

 

First of all you have a few things mixed up.

1st: You cannot bore a head. This is done in the cylinder. You can port the head though, this is done for flow where-as boring is usually done to gain displacement and clean up imperfections in the cylinder wall during a rebuild (an oversize piston is required after boring).

2nd: You cannot stroke a top end. The top end of a motorcycle engine is usually considered the cylinder, cylinder head and associated parts. If you want to stroke the engine you have to get a crankshaft (which resides in the lower end).

I have seen alot of posts regarding 350X parts on other engines, but have seen little good info actually detailing how this process is done, eg. special piston wrist pin location and other considerations.

You would be better off just going with a 350X engine and not messing with any of the other stuff. Instead of spending money on having a 300EX or 250X stroked, the 350 is essentially already stroked. Another benefit is that the 350X will have a stronger lower end (transmission, clutch & basket, crankshaft & bearings) and the reliability will be better. The 350X is larger than the other combinations as well. Here are some stock stats:

250X : Bore 74mm X Stroke 57.3mm = 246cc
300EX: Bore 74mm X Stroke 65.5mm = 282cc
350X : Bore 81mm X Stroke 68.0mm = 350cc

Here are the stats for various engines with the 350X topend (if even possible)

250X : 81mm X 57.3mm = 295cc
300EX: 81mm X 65.5mm = 337cc
350X : 81mm X 68.0mm = 350cc

It's pretty plain to see why you would be much better off by starting with a 350X, modifying it to your liking and not messing with trying to mix and match it with a smaller engine.

As far as trying to bore it or any other engine to 425, you must keep in mind you can't just bore any engine you like as far as you like, there are limitations. The largest piston offered by Wiseco for the 350X is 82mm (.040 over) this brings the total size to 359cc. Of course you can probably find a big-bore kit, but this involves replacing the stock cylinder and spending a good chunk of change.

A 350X will need some pretty heavy modifications to be able to outrun a modified 250R in a drag race. The power comes on much differently though and is more controlable, and you may be much faster on the 350X even though the 250R is a stronger engine depending on what kind of riding you are going to be doing. You are correct to think that a 350X powered machine would be a great setup for trail/woods riding and CC racing.

Gabe
'86ATC250R
'86ATC350X
'00TRX400EX

[This message has been edited by 86atc250r (edited 09-01-1999).]

 

om a different site i saw a pic of a RX conversion bored to 435. I messed up on the other stuff cause i waz half asleep. I have another question i want to ask but can't think of it right now. Thanks

 

Be careful man

A 350x motor in a R chassis does involve some work. I am not sure if you are aware of this yet or not. The bottom frame rails will have to be dropped and widened, OR the swingarm pivot bolt will have to be moved up along with linkage of course.

I am in the process of doing one of these conversions now.

As far as the 350x motor is concerned. Powroll has a big bore kit for the 350x that does not require the cylinder to be resleaved. The kit comes with an 83mm piston and you send your crank to them and they add 6mm to stroke it. Then end result will be a 400cc motor and according to them a 40% increase in power. Since Honda says that the 350x motor puts out around 27 horses (doubtfull) a 40% increase would yield about 10 extra ponies. The cost? Well look around spending about $500.00 not including shipping or having someone split your cases. Unless you can do it yourself.

Good luck (to you and myself cause I know I am going to need it)

 

I will proably ship the R frame out to rocky ridge racing. The do some work on the frame so a 350x can be put in.

What comes with the 400 kit?

 

Should i buy a used to 350x to get the engine or should i buy a new 350x engine from a place like east coast atv, for like 1300?
By me i can proably get a good 350x of rlike 500

 

In reply to Brad_L: I don't doubt at all that the 350X motor put out 27 hp at the crank. A stock 400EX puts out about 28 at the rear tires, so you have to figure some loss in there. The 350X has always been a very good running engine, has a much different powerband than a 400EX. I think my old 350X could run right with my 400EX (75 lbs lighter, but I haven't been able to race them as my 350X is torn down right now)

In reply to MXER: If it were me, since you plan on going thru the engine anyway, used would be the only smart choice, no sense in tearing out brand new high dollar stuff to replace it with more high dollar stuff. Just look around a little and try to find one that looks like it has had an easy life.

Sounds like a fun project guys, good luck, let us know how they turn out.....

Gabe
'86ATC250R
'86ATC350X
'00TRX400EX

 

thanks man, i proably won't do this to like next year or something. need money

 

Hey MXER,

Have you talked with Ken at Rocky Ridge? Has he give you a price on modifying the chassis? I talked with him a few months ago about doing mine.

The way you make your 350x a 400 actually requires two different modifications. The stroked crank which by itself would make the bike a 373cc. Then the big bore piston which adds the remaining cc's. If you go any bigger than a 83mm piston you will have to resleave.

I agree with 86atc250r about the motor. If you are going to tear it down and go bigger. Keep a lookout for a used motor. The main thing to look out for on a 350x motor is the head. If the head is damaged due to the camshaft then look for another motor. The head on the 350x is very expensive.

86atc250r
That motor may just make about 25 to 27 horsepower stock. I wish you could ride your 400 then immediately ride the 350 and give us a comparison. I have never rode a 400ex. I used to have a 350x three-wheeler though. Although all that is left of it is the motor which is mounted in my 1991 250x with 250r suspension.

Now if I can just get my 350rx running I will be happy.

 

Hopefully I will be able to ride the two and compare soon, I am currently rebuilding the 350X so that I can sell it to a friend (it's down to a bare frame right now). If my memory serves me correctly the 350X had a lot more off the very bottom, the mid's are pretty equal, and the 400 having a little more on top. My stepfather has an '85 350X in running condition, I may get it out soon and take it for a spin, I don't really like it, though due to the twist throttle (he bought it that way, neither of us likes it). It's motor is a little worn out too, so it may not be a fair comaparison. The only thing that was done to mine, was that I removed the spark arrestor and bottom cleaning plate, free's up the exhaust quite a bit and sounds much better, can't really do that on the 400EX, it sounds pretty nasty without the spark arrestor. The other thing that makes the comparison unfair is how well the 400 chassis hides it's speed, you don't realize how fast you are going until you almost get yourself into trouble.

I am going to email L.Sommers at www.geocities.com/~gtthunder and see if he has a dyno run on the stock 350X. That will tell the tale.

Gabe
'86ATC250R
'86ATC350X
'00TRX400EX