Rule changes for F4

**F5 Dave** · 11th April 2011, 17:01

I've only just seen this thread

I only really have opinion on 1

Many of these engines are getting worn. Pistons are getting harder to get. In a similar vein that the recently changed F4 restriction for 150cc 4 strokes was increased to more oversizes (158.09cc) it is suggested that the current 104cc restriction is increased to 106.2cc.

This would allow a popular, cheap & easily available 52mm KT100 piston to be used in several models (TS100, TF100, GP100, MB100, H100, A100 etc) without having to destroke the crank (an expensive exercise).

Additionally amend:
F4 2 stroke engines over 106.2cc are restricted to carburation equivalent to a single 24mm carburettor

So that would increase the avail cc of a 100cc 2 stroke 2.6cc. How bad is that?

And why should 4 strokes get an increase if 2s can't?

Otherwise you need to get a destroked crank to run a KT piston. Speedpro would be a little sore as he has had to pay this.

**kel** · 11th April 2011, 17:27

Originally Posted by F5 Dave

In a similar vein that the recently changed F4 restriction for 150cc 4 strokes was increased to more oversizes (158.09cc) it is suggested that the current 104cc restriction is increased to 106.2cc

What is this, find a cheap piston and change the rules to suit?
I cant find a reasonably priced 24mm carb for the 125 so I propose we do away with the 24mm carb restriction, actual I cant find a suitably priced piston either so Ill take the extra 6.2% as well. 132.75cc breathing through a 35mm PWK, no advantage in that surely.

The old 2 strokes are tired and there wont be new material anytime soon, but there are plenty of after market parts available so play within the rules. If it aint broke dont fix it.

**F5 Dave** · 11th April 2011, 17:48

Not at all, and usually I'm the staunchest 'don't fix it' proponent.

but in this instance I think its a small change that allows an easily available piston to be run. A few are running this piston now, but not many have the resource or will to de stroke a crank.

This saves a few beyond-last-size barrels, & lets face it; all the 2 stroke 100s are 30 years old now. They've been past 1st & 2nd OS. Here's another. The 150s were given another oversize option & they aren't 30 years old yet.

oh hold on that's 2.2cc over current, not 2.6

**Bert** · 11th April 2011, 18:22

Originally Posted by F5 Dave

I've only just seen this thread

I only really have opinion on 1

Many of these engines are getting worn. Pistons are getting harder to get. In a similar vein that the recently changed F4 restriction for 150cc 4 strokes was increased to more oversizes (158.09cc) it is suggested that the current 104cc restriction is increased to 106.2cc.

This would allow a popular, cheap & easily available 52mm KT100 piston to be used in several models (TS100, TF100, GP100, MB100, H100, A100 etc) without having to destroke the crank (an expensive exercise).

Additionally amend:
F4 2 stroke engines over 106.2cc are restricted to carburation equivalent to a single 24mm carburettor

So that would increase the avail cc of a 100cc 2 stroke 2.6cc. How bad is that?

And why should 4 strokes get an increase if 2s can't?

Otherwise you need to get a destroked crank to run a KT piston. Speedpro would be a little sore as he has had to pay this.

I agree with you here Dave. very valid points.

Originally Posted by kel

I cant find a reasonably priced 24mm carb for the 125

I'll do you a deal: 24mm taiwan PWK ~$95.00 (not a KOSO; but an OKO; non powerjet). or these guys can be an option if they can get stock (140 with powerjet):
http://shop.scootling.co.nz/index.ph...&productId=606

Originally Posted by F5 Dave

but in this instance I think its a small change that allows an easily available piston to be run. A few are running this piston now, but not many have the resource or will to de stroke a crank.

This saves a few beyond-last-size barrels, & lets face it; all the 2 stroke 100s are 30 years old now. They've been past 1st & 2nd OS. Here's another. The 150s were given another oversize option & they aren't 30 years old yet.

I've got a shed load of "beyond last oversized" barrels waiting for this day... Kart pistons are the best option for what we are actually trying to do with these motors (making reving race bikes).
Though the KT's start at 52mm there are other Kart pistons that start well below that; if you are willing to go hunting; a little more expensive but not as much as a RG400 piston.

and actually made for racing applications.

Its been a year at lease: How about pre powervalved MX80s with a 20mm carb restriction???

**Yow Ling** · 11th April 2011, 19:15

I voted against the changes, so rather than go edit my post, ill make a new one, speaks with forked tongue !

I am building a new 100cc engine, selecting a piston has been a nightmare, but I was ok with that because I thought it was fair and went into it with my eyes open.

Ive found a piston that will do at about $200 a pop, a much cheaper kt100 piston would give tons of options plus a few extra cc wouldnt go astray , a thinner sleeve would be an advantage.

The downside of allowing 52mm pistons is that the intention of saving the old mbs will allow 35hp engines at the other end, this suits me but i was happy to play under the old rules.

the big carb 125 stuff has been done to death

it is a bit of an anomaly that the fxr guys have 8 rebores available and skip over the first 7 to go straight to +2.00mm. the 2 strokers get about 4 rebores and dont have the luxury of wasting rebores given the investment in the porting

This has given rise to the 156 or so cc standard. One racer was arguing that a long stroke std bore was legal if the capacity was the same as the overbored std cylinder, the rules are specific regarding allowance for reboring.

Maybe instead of allowing the 100cc engine to grow , we need to curb the generosity of the diesel reboring rule

**speedpro** · 11th April 2011, 19:22

Dave makes some good points though I wouldn't be too annoyed about everyone getting to use KT pistons without the cost. I have a few bits I could use to make more engines myself. It isn't the oversize so much as the extra revs using those pistons allow that I'm hesitant about.
Looking back I used to replace the piston in my TS100 every two meetings with another RG400 piston. That added up pretty quickly.
The only reason I'm using the KT piston now is for more revs. The MB pistons, or TKRJ, all last well and the cost spread over 4-5-6 years is neglibible. FYI - the MB pistons are good for a reliable 13,000rpm and the rings will only lose a tip now and then over 14,000rpm.

That does look a bit like I don't want anyone else getting to use the pistons I'm using.

**speedpro** · 11th April 2011, 19:25

Really I'm looking at the little increases sneaking into each class/category. Where does it end??

**saxet** · 11th April 2011, 19:39

Well I'm ambivalent about suggestion 1 but still not for No.2.

**bucketracer** · 11th April 2011, 20:01

Speedpro de-stroked his crank with an offset pin, so he could use a 52mm piston and stay within the 100cc limit.

With the ready availability of CNC machining, changing the stroke would not be that hard. ESE pay about $160 a crank pair to have the big end holes of their GP's bored out from 19mm to 22mm for better RGV250 rods/pistons and resultant RPM. It wouldn't be that hard for the pin holes to be off set at the same time if they needed to be.

I wonder how many people have actually taken a set of crank halves to their local engineering shop and asked them about the possibility's and cost.