RD125LC barrels on a GT380?

[sportston]

Has anyone ever tried to fit a set of RD125LC barrels on a GT380?
I know about the Barton 380.
I was toying with the idea of doing something similar using an early set of RD125LC barrels. Then I could fit reed valves and have over 63bhp without even getting them big bored! Ooooh yeah!!!
Of course, matching the top end to the crank might be difficult. I have a feeling it could be expensive, upwards of £2000, but wouldn't it be awesome?!
I wonder how many knots it would tie the standard GT380 frame into. Lol

[dollydog]

see my reply to your other thread. whatever you do, take your time, think it out and enjoy
if i was doing this i'd put it in a t500 frame - so much stronger
cheers, dd.

[sportston]

see my reply to your other thread. whatever you do, take your time, think it out and enjoy
if i was doing this i'd put it in a t500 frame - so much stronger
cheers, dd.

You suggested rg250 barrels but they will not fit. All the porting is wildly different on the rg125 and rg250 barrels. I looked into that and I wouldn't have a hope of making them fit. But the RD125LC has a barrel that looks like it may fit with a little modification.
Your other suggestion is an interesting one of using GT250 liners and boring them out. Maybe I will do that one day. We shall see...

[dollydog]

no mate, i never mentioned rg250 barrels - i said rg150 but that was simply for the bore size. wildly different just makes things slightly more complicated, but more job satisfaction in the end anything can be made to fit anything, given time, patience and a big writing pad and several biro's oh, yes, money sometimes helps
cheers, dd.

[Craig380]

It would probably need some careful calculations as the 125LC motor has a 50mm stroke, and the pistons probably have different heights from gudgeon pin to the crown .... so you'd probably need a spacer plate if you keep the standard 380 crank and rods.

It would be interesting to offer up a 125LC barrel to the 380 cases to see if the studs & stud holes are close, and if the transfer troughs in the cases get anywhere close to the transfer mouths in the barrels. I wonder what diameter gudgeon pin the LCs have too.

[tz375]

That sounds like a really interesting project.

For teh frame, I would forget the 500 or 550 frames which are heavy and not all that stiff. Look at what you can do to your frame to brace it first or build a new frame in chrome moly tubing...

[sportston]

It would probably need some careful calculations as the 125LC motor has a 50mm stroke, and the pistons probably have different heights from gudgeon pin to the crown .... so you'd probably need a spacer plate if you keep the standard 380 crank and rods.

It would be interesting to offer up a 125LC barrel to the 380 cases to see if the studs & stud holes are close, and if the transfer troughs in the cases get anywhere close to the transfer mouths in the barrels. I wonder what diameter gudgeon pin the LCs have too.

Yes I'm curious to know that too. Don't fancy spending the money on buying one and trying it yet though.

[sportston]

That sounds like a really interesting project.

For teh frame, I would forget the 500 or 550 frames which are heavy and not all that stiff. Look at what you can do to your frame to brace it first or build a new frame in chrome moly tubing...

In an ideal world I would like a special frame but OMG they cost a lot! A Seeley would be interesting

[kb75ts250]

The GT380 can be ported to TM125 specs (width not height). When I built mine that's the way I did it, and used 3 TM125 carbs, and cut the head apart and milled the outer 2 to match the TM125 cc volume. Left the center head alone due to concerns with cooling. I can't recall the brand of chambers I used but they were short and fat, as close to TM125 dimensions as I could find. It ran quite well and wasn't very difficult to do.

[sportston]

The GT380 can be ported to TM125 specs (width not height). When I built mine that's the way I did it, and used 3 TM125 carbs, and cut the head apart and milled the outer 2 to match the TM125 cc volume. Left the center head alone due to concerns with cooling. I can't recall the brand of chambers I used but they were short and fat, as close to TM125 dimensions as I could find. It ran quite well and wasn't very difficult to do.

Sounds interesting. I'd never even heard of a TM before. But then I've never been into off-roading. Watched a video on youtube and the TM looks a pretty quick bike. Which TM125 did you mean? The rapid modern liquid cooled one or the old Suzuki TM125 from the 1970s?
What sort of bhp did you get out of your GT380 after you had ported it? Was fuel economy bad? Did you have any problems with overheating?
What were the port dimensions you used?

[kb75ts250]

I'm referring to the older 125 mx bikes from the early 70s before they went to reed valves and water cooling. They were putting out around 18-20 hp so I figure my 380 was close to triple that. In terms of performance, it was at least as good as a Kawasaki H1 (but my 380 did have a weight advantage). Don't recall the fuel economy (or lack thereof ha ha).

I do not have the port specs any more but back then they were pretty readily available. I worked in a shop so it was also easy to take my own measurements from a TM125 cylinder, then I extrapolated to correct for the bore/stroke difference between the TM125 and GT380. Another very easy option would be to look into historical T250 hop-up info. If I recall correctly, the bore, stroke, and many crank and top end parts were identical between the GT380 and T250/GT250. T250s were popular with the road racers and there were lots of aftermarket race parts available. I also did a crank rebuild using Rotax (Can-Am) snowmobile engine bearings, thrust washers, wrist pins and I think the rods too. I can't recall exactly which Rotax engine.

The first year 380s had issues with the center cylinder running hot. We used to switch to a one step colder plug for the middle cylinder before delivery. The whole reason I cut the heads apart was so I could mill the outside ones but leave the center one alone. I also made a sheet metal ram air shroud with a taller and angled opening for the center cylinder.

[sportston]

I'm referring to the older 125 mx bikes from the early 70s before they went to reed valves and water cooling. They were putting out around 18-20 hp so I figure my 380 was close to triple that. In terms of performance, it was at least as good as a Kawasaki H1 (but my 380 did have a weight advantage). Don't recall the fuel economy (or lack thereof ha ha).

I do not have the port specs any more but back then they were pretty readily available. I worked in a shop so it was also easy to take my own measurements from a TM125 cylinder, then I extrapolated to correct for the bore/stroke difference between the TM125 and GT380. Another very easy option would be to look into historical T250 hop-up info. If I recall correctly, the bore, stroke, and many crank and top end parts were identical between the GT380 and T250/GT250. T250s were popular with the road racers and there were lots of aftermarket race parts available. I also did a crank rebuild using Rotax (Can-Am) snowmobile engine bearings, thrust washers, wrist pins and I think the rods too. I can't recall exactly which Rotax engine.

The first year 380s had issues with the center cylinder running hot. We used to switch to a one step colder plug for the middle cylinder before delivery. The whole reason I cut the heads apart was so I could mill the outside ones but leave the center one alone. I also made a sheet metal ram air shroud with a taller and angled opening for the center cylinder.

Interesting stuff. thanks for your input. I have already made some efforts to help the centre cylinder. I drilled a few tiny holes in the cooling fins to create a little turbulence.
When you say you milled the head, do you mean you skimmed the gasket surface? Or did you mean you altered the profile of the compression chamber?
So you widened the ports but did not increase the height of them?

[kb75ts250]

Skimmed the gasket surface of cyl 1 and 3 to match the measured volume of the TM125 combustion chamber as closely as I could without cutting into the dome. If I recall correctly most of the work was widening with minimal height increase, but as I said before I did it extrapolation, so the width degrees matched. Also opened up the whole intake port and rubber adapters to match the bigger carbs and matched the transfer ports in the upper crankcase to those in the cylinder.