4 replies to this topic

[Bob Riebe]

I had thought about putting this in the tech. area but as it is a 60 some years old article, I put it here.

 

This article from Cycle World :

Harley-Davidson Kr-Tt & Sprint Cr-Tt

 

https://magazine.cyc...tt-sprint-cr-tt

 

 

 

A curious item; in another site where they spoke of taking the KR engine to Axtell for head work, in 1967, he took one of his engines , removed the Tillotsen, carbs., put on two Mikuni and got a 3hp increase.

After Axtel's work, they got 60 some HP, at the rear tire, out of the engine.

Edited by Bob Riebe, 27 November 2024 - 18:54.

[GregThomas]

I think you're getting the KR and the CR mixed up. KR 750 peaks around 7500. CR 250 can go to 10500.

 

The CRTT was based on the Italian Aermacchi 250/350 single family. I've built quite a few of those for modern Classic racing.

It's fairly simple to upgrade the HD Sprint engine to CR performance levels. They're a well designed pushrod OHV single.

Interestingly they're quite sensitive to stroke length - ie piston speed.  The 250's used at least 3 different strokes and the 350's 2.

The valve train is the same all the way through and with modern springs/Ti retainers is stable to very high rpm.

But while the short stroke 250's will go to 10500rpm, the 80mm stroke 350 is reluctant to go past 8500/9000 rpm.

Edited by GregThomas, 27 November 2024 - 18:37.

[Bob Riebe]

You are correct, my error, I had forgotten about the 4 stroke Aermacchi.

I remember the 2 stroke bikes, so when I read valves I some how assumed this was a flat-head.

 

Sad when as time goes by you forget more than you remember.

 

This way the thread title will get more people looking.

Thank you.

Edited by Bob Riebe, 19 December 2024 - 19:57.

[Bob Riebe]

https://www.mecum.co...rtt-road-racer/

 

https://images.mecum...8/735060_1.jpg?

 

This is a KRTT built for an independant racer, who never raced it.

The rear foot-peg was installed incorrectly or the rider likes the smell of burnt rubber.

This is for sale at the Mecum auction, which after looking I think has a lot of very rare early racing/street  motorcycles. I wonder if some of those are not being flipped from the Auction of the closed National Motorcycle Museum in Amarosa , Iowa.

 

I wonder how many more HP they could get out of KRTT engine using CFD software, and modern carbs.

 

 

Edited by Bob Riebe, 19 December 2024 - 19:58.

[GregThomas]

The last works bikes used dual Mikuni VM38 carbs. Still pretty much state of the art for slide carbs.

 

While i agree CFD may show a possible improvement, these things were developed for so long that pretty well 

everything you can imagine was tried.

 

I've read 66-68 Hp at the crank. Which isn't bad for a flathead.

 

I'd happily have that one.