XR75 Build THread.... The OG midsize

[Cactus Jack]

Check out the price people will pay for a re-pop / COPY that they're claiming to be a genuine DG finned valve cover ...

honda xr75 xr75 dg finned aluminum valve cover ahrma: eBay Motors (item 300488778608 end time Nov-09-10 17:49:51 PST)

Info on how to make your own XR performance pipe ...

need help selecting tubing dia. & pipe for custom exhaust - ThumperTalk

I have a new DG XR75/80/100 snake pipe (up pipe) and it measures :

1) - 1 1/8 inches diameter thru the "U" turn out of the head for 11 inches .

2) - then it steps up to 1 1/4 inches for the next 23 inches .

3) - then it goes into a reverse cone megaphone "muffler" that is 12 inches long up to the widest point of 3 inches in diameter - then it tapers down over the next 1 1/4 inch into a 2 3/8 inch diameter x 4 inch long tail piece .

4) - the inner diameter of the perforated megaphone core is 1 1/4 inches all the way thru

5) - the machined header flange measures :

7/8 inches overall length x 1 1/16 inches I/D

and is 1 3/8 " O/D x 3/8 " long stepping down to 1 3/16 " O/D for 1/2 " long .

Since your engine is bigger than 100 cc - I'd say that 1 1/4 won't hurt anything . Also , a crush in on the inside of a "U" turn bend won't hurt flow because the gas wants to flatten out and spread like a "D" shape on the outside wall of tight bends .

Stock early XR 75 header pipes are 1 3/16 inches in diameter so the DG pipe is 1/16 of an inch smaller out of the head . The megaphone and step up in the pipe sucks the exhaust gas out at a higher velocity than a parallel pipe - and increases power right throughout the entire rev range .

MCM SL125 /145 header pipes were 33 mm - nearly 1 5/16 inches

I converted mm to inches at the time I posted the info to make things easier for the Yanks ...

Incidentally a Powroll XR75 power pipe is 32 mm O/D ...

 

[The_GDFP]

Head & barrell are off,

Inlet valve is chewed out nearly pulled through

every thing else looks to be in great condition.

will throw up some picks after work

Later XR inlet valve is 1mm bigger, a nice way to save replacing the seat in the head, and feed a bigger motor.
BTW, how much over 100cc, and how'd you go aboot it?

 

[The_GDFP]

Inches are for carpenters, respectable bikes are Metric, or Whitworth :bocul:

Good info though, Cactus

Where's the rest of this build, Hillz? Those 80-81 CR80 shocks aren't TOO far off for period-correct Maybe closer on my '77. Better ship 'em over and Turtle a set of Boge's for your early model, eh?

 

[Cactus Jack]

I bought a set of new Boge Orcas off Tzflynn in the US and they're good ... but they're heavier than the original orange Boges ...

 

[The_GDFP]

These ones? I'm down to a few sets of short ones. All the flat-trackers snatched the good sizes. 275mm is aboot all I have left.

 

[Cactus Jack]

Yeah they're lighter than the Boge Orcas ... The Orcas are 12" eye to eye (330 mm) ...

 

[TyD]

PS if any one has a side plate for XR75 let me know, i'm not paying $200 for a set... id rather make one

You want the yellow plastic ones that were on the bike originally? I got a set of 3 here if you're keen.

 

[The_GDFP]

Yeah they're lighter than the Boge Orcas ... The Orcas are 12" eye to eye (330 mm) ...

330 is closer to 13". I thought your Aussies went both ways?:aetsch:

 

[Cactus Jack]

It's always better to say you've got an inch less than you actually have ... Adds to the surprise ...

 

[The_GDFP]

lol. That'll make the difference from "ooh" to "AAAAH!", eh?

 

[Cactus Jack]

There ya' go ... A set of painted up $50 USD Boge Orca's ... According to him they're 12.5" eye to eye ...

honda xr75 xr80 boge shocks vintage dirt bike ahrma - eBay (item 300496241323 end time Nov-29-10 18:52:05 PST)