eligible pre 75 honda

[35elsinore]

Hi knowledgeable ones. I am looking for a sl/xl 125 engine that is pre 75 legal, two engines have cropped up but have two piece heads that I believed is not pre75. The lastest engine has raced aussie titles numerous times in dirt track pre 75 class and passed scruiteneering. I hope theres a simple answer without to many varied opinions.
Cheers Troy

[35elsinore]

bump

[matcho mick]

wonder if retro fitting earlier 1 piece head to those later motors will satisfy iledgibility for pre75?,might have a head kicking about!,

[35elsinore]

Thanks for the offer mick. Ill wait and see if anything is on offer at CD.

[Barronvmx]

I race a XL100 and SL100 and both have 1 x piece heads and the original gear box. The 1975 models bottom ends will give you no advantage and are not pre 75 so don't bother. The CB100 and SL100 bottom ends have the best gear ratios and boring out to 125cc with a cam should be enough. I raced the XL100 on standard bore and was able to get mid pack with small modifications. The high compression 125 piston goes in over the off season and a better cam so I might have a Elsinore beater soon.

[bigtoe]

hi, both my xl125's have two piece heads and I've been racing them for years in pre 75 at club, state and national titles and no one has ever said anything at scrutineering as I don't think they offer any advantage on the track, only ease of rebuilding

[Tim754]

Big toe that is a no no. If anyone put up the protest monies you would be gone. No two piece heads are legal in pre75. The list of eligible machines in the Manual  of Motorcycle Sports Australia used to (maybe still does ) Have this fact stated. Yes a bit easier to work on but they are pre 78 or pre 80 class. SL125 is fine but XL and CT 125s nope ,SL100 and  very early single piece XL100 yep good to go too and  your right there be be SFA advantage in go power but that is the way it swings.
SL125 Race bike builder for over 35 years and former VMX Steward ,CoC , eligibility Scrutineer.  Tim754

[bigtoe]

Hi tim, yes you are completely correct on that point, I've just returned from holidays and found that someone has used my computer on this forum and mentioned something about two piece heads ? I didn't even know they made them ! I' m going down to the shed right now and check, if by some  chance they have broken into two pieces well I'll just weld them back into one, so there.
ps - hey tim did you get that $200 bucks I sent you

[Daryl Jones]

The '76 XL 2-piece head has valves that are several mm larger than the early one.
It would cost serious money to fit bigger valves to the early head.
(machining req'd to offset new seats & guides, etc).

Apparently, the 2-piece head raised the power of the stock engine from 6hp. to 7hp.
Doesn't sound like much, but that's about 17% with the same small carby, a restrictive exhaust, etc.
Some of that might be due to the electronic CDI ignition.
(Not permitted by AHRMA for pre'75, Points ignition only, Not policed in Oz).
I reckon that an extra 17% hp in my race engine would be 'helpful'.

If 2-piece heads were permitted, there would be nothing stopping you from fitting an xr200 head with it's relatively huge valves.
While we're into "flow on" type upgrades you might as well fit an xr200 crank, cos they are easier to find and get bearings & rods for too!

If you can squeeze the 6-speed gearbox into Pre-75 cases, I have no problem with that. Do a post and show how it's done.
Use pre-75 engine cases and a one piece head and you'll hear no protests from me.

My VMX/DT/Vinduro race bike is a '72 SL100 with a special CB125 piston (14mm pin).

[Tim754]

ps - hey tim did you get that $200 bucks I sent you       Um no I don't think so, more info required thanks mate?

Daryl (above)  has very impressively and neatly summed up the situation. Using a 14mm pin piston increases the available parts too.