PICTURE_MAN36 Posted January 28, 2009 Share Posted January 28, 2009 Hello Can someone possibly dispell a popular myth for me. I have an 83' Y DT125MX which I am currently restoring. When we were in our teens, we were always told that the DT125MX & DT175MX shared the same bottom end, and that the barrels could be interchanged, i.e, we could put a '175' barrel on a '125' bottom end and v.v. Is this true? I've spoken with Yamaha who say no, but I've also been told by many previous owners that this is possible. They've never done, but always know someone who did, and in the immortal words 'Cor! it went like stink afterwards'. I've never actually ridden a 125MX, but the popular concensus I have been given by a number of people who did, says that the 175MX is quicker excellerating, and does not run out of breath as quick as the 125MX I do know that it is possible to drop a 175 engine in a 125 frame, but I understand that you need to change the carb, the ecu, and both front and tail pipes, and I guess there is no way that these items will be available at a reasonable price anymore. Any info would be grately appreciated.. Thank you Link to comment Share on other sites More sharing options...
JimR Posted January 28, 2009 Share Posted January 28, 2009 Hello Can someone possibly dispell a popular myth for me. I have an 83' Y DT125MX which I am currently restoring. When we were in our teens, we were always told that the DT125MX & DT175MX shared the same bottom end, and that the barrels could be interchanged, i.e, we could put a '175' barrel on a '125' bottom end and v.v. Is this true? I've spoken with Yamaha who say no, but I've also been told by many previous owners that this is possible. They've never done, but always know someone who did, and in the immortal words 'Cor! it went like stink afterwards'. I've never actually ridden a 125MX, but the popular concensus I have been given by a number of people who did, says that the 175MX is quicker excellerating, and does not run out of breath as quick as the 125MX I do know that it is possible to drop a 175 engine in a 125 frame, but I understand that you need to change the carb, the ecu, and both front and tail pipes, and I guess there is no way that these items will be available at a reasonable price anymore. Any info would be grately appreciated.. Thank you Hi it is possible to put a 175 motor in a 125 frame but adding a 175 barrell & piston to a 125 crankcase may cause a problem whats an ecu ? the bike only had a CDI mag and CDI unit ... I would guess the CDI unit was the same for both, as their ignition curves are the same. I guess that if you mean a programable engine management system (ecu) these bikes didnt have one . The CDi just advanced or retarded the ignitin timming according to engine rpm if you need more info send a reply Regards Jim Link to comment Share on other sites More sharing options...
Caramba Posted January 28, 2009 Share Posted January 28, 2009 Hello Can someone possibly dispell a popular myth for me. I have an 83' Y DT125MX which I am currently restoring. When we were in our teens, we were always told that the DT125MX & DT175MX shared the same bottom end, and that the barrels could be interchanged, i.e, we could put a '175' barrel on a '125' bottom end and v.v. Is this true? I've spoken with Yamaha who say no, but I've also been told by many previous owners that this is possible. They've never done, but always know someone who did, and in the immortal words 'Cor! it went like stink afterwards'. I've never actually ridden a 125MX, but the popular concensus I have been given by a number of people who did, says that the 175MX is quicker excellerating, and does not run out of breath as quick as the 125MX I do know that it is possible to drop a 175 engine in a 125 frame, but I understand that you need to change the carb, the ecu, and both front and tail pipes, and I guess there is no way that these items will be available at a reasonable price anymore. Any info would be grately appreciated.. Thank you I can't asnwer your question from a position of definitive knowledge but applying a bit of logic the answer I would arrive at would be no unless you change more than the barrel. If as the anecdotal evidence suggest such a conversion would cause the bike to "go like stink" then one assumes that the capacity of the engine has somehow increased. There are only two factors that determine that, the bore and the stroke. If the 175 and 125 have the same bore then there isn't an increase in capacity, you're replacing like for like. If however the 175 has a bigger bore then the 125 piston is going to rattle around inside it and of course the engine won't work. If the bore is the same but the barrel is a bit longer in the 175 to give it the extra capacity then fitting it to a 125 bottom end will do nothing because the stroke is controlled not by the barrel but by the profile of the crank. Your swept volume (engine capacity) remains the same. So if the 175 barrel is longer than the 125 barrel (but same bore) you'd need a new crankshaft too. If the 175 barrel is the same length but bigger bore you'd need bigger pistons to fit the bigger bore. I don't know the engine but it's possible that the difference between the two is in both bore AND stroke so you're doubly stuffed. I'd find the specs for the two engines and get the bore and stroke dimensions. UPDATE Search in the Internet reveals that the 125 has a bore size of 56mm and the 175 is 66mm so your 125 piston would need changing. The stroke size however is identical at 50mm. So it suggests the bottom ends (including crank) are the same dimensionally. The wider barrel would of course require you to throw away the 56mm 125 piston and buy a 66m 175 one. The only problem then is whether the bigger piston will fit the top of the con rod. It's unlikely that the gudgeon pin will be different but that's something I can't tell you. Furthermore there's the question of whether the cylinder heads are identical and whether the bottom end bearings are stronger for the higher capacity engine. At least we have an answer. It isn't a straight swap as the pistons are different. Link to comment Share on other sites More sharing options...
Caramba Posted January 28, 2009 Share Posted January 28, 2009 <sheepish face on> PICTURE_MAN it's clear from reading some of your other posts that you're more than a little experienced at this game and consequently everything I've written above is glaringly obvious to you. I now fell a bit sheepish but was only trying to help. <sheepish face off> Link to comment Share on other sites More sharing options...
PICTURE_MAN36 Posted January 29, 2009 Author Share Posted January 29, 2009 Thanks for the replies. In the first instance my apologies. I had my car head on at the time. I meant to put CDI not ECU (This is what comes of trying to have a sneaky look at a forum, whilst at work, when your supposed to be being productive!!!). I have the Haynes manual for the DT125/DT175MX and it lists the part numbers, bore & stroke etc. There were many models made, for many countries, and I would guess that looking in the book a good 85 / 90% of the bits are common, with the exception of the engine, carb, exhausts (both front & tail) and the CDI unit I believe. I guess I am better sticking with what I know works. If I decide that the 125 is a bit gutless, I might look to swap the complete engine for a 175 engine. It would seem that the 175 is more common than the 125. (the model I have was the final variant of the air cooled motor before Yamaha went to liquid cooling). As with most older machines (I won't use the word classic so I don't offend the old gits on the nice BSA's and Royal Enfields that always seem to be broken down at the side of the road in a pool of oil), finding good parts is getting harder and harder and I didn't want to spend potentionally £ 100 plus on a barrel, head, piston, rebore etc, if the myth was a load of old tosh!! When completed, it will be only be used locally anyway (gone are the days when I want to do 1000's of miles on a 125, screaming along the road at 69 mph, flat out!!!) but I had considered rebuilding the top end with 175 bits if the myth was true. I think in essence, what actually happened in the past was someone sneaked a 175 engine in a 125 frame, and made all of their mates look naff on their stock 125's. Does anyone remember anyone doing this with the RD125MX & RD200MX???? They were identical, but for the fact that the 200 had an electric start! Thank you for your responses. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted January 29, 2009 Moderator Share Posted January 29, 2009 I have the Haynes manual for the DT125/DT175MX and it lists the part numbers, bore & stroke etc. There were many models made, for many countries, and I would guess that looking in the book a good 85 / 90% of the bits are common, with the exception of the engine, carb, exhausts (both front & tail) and the CDI unit I believe. I guess I am better sticking with what I know works. If I decide that the 125 is a bit gutless, I might look to swap the complete engine for a 175 engine. It would seem that the 175 is more common than the 125. (the model I have was the final variant of the air cooled motor before Yamaha went to liquid cooling). As with most older machines (I won't use the word classic so I don't offend the old gits on the nice BSA's and Royal Enfields that always seem to be broken down at the side of the road in a pool of oil), finding good parts is getting harder and harder and I didn't want to spend potentionally £ 100 plus on a barrel, head, piston, rebore etc, if the myth was a load of old tosh!! When completed, it will be only be used locally anyway (gone are the days when I want to do 1000's of miles on a 125, screaming along the road at 69 mph, flat out!!!) but I had considered rebuilding the top end with 175 bits if the myth was true. I think in essence, what actually happened in the past was someone sneaked a 175 engine in a 125 frame, and made all of their mates look naff on their stock 125's. Does anyone remember anyone doing this with the RD125MX & RD200MX???? They were identical, but for the fact that the 200 had an electric start! Thank you for your responses. I ve been watching this thread and like you I have heard that the 125 barrel head and piston...and carb, can be swapped for the 175 versions, it seemed to me that the only thing to check is the part numbers for the crank cases, if the cases are the same, and as you have stated before the crank / con rod are the same, then this would be a good proposition, a quick look on Bike Bandit site confirms that both engine sizes use the same crank cases, therfore, theres nothing stopping you from putting the 175 barrel and carb on to your 125...is there? you can see the part numbers below. ...Paul Link to comment Share on other sites More sharing options...
PICTURE_MAN36 Posted January 29, 2009 Author Share Posted January 29, 2009 Thanks for your response. I need to make a list now I think. I guess other than the obvious things like barrel, head, piston, rings and little end bearings, I'm looking for a 175 carb as i believe that the carb listed has a different part number. I'm also looking for a front pipe, tail pipe, and may be a CDI unit? (I'm sure that the guy at Yamaha said that the ignition timing was different. I guess that it would be pointless sticking with my carb, front and tail pipe as the '83 MX would have been restricted to 12 BHP by the carb and exhaust? Thank you Link to comment Share on other sites More sharing options...
Moderator Airhead Posted January 29, 2009 Moderator Share Posted January 29, 2009 Thanks for your response. I need to make a list now I think. I guess other than the obvious things like barrel, head, piston, rings and little end bearings, I'm looking for a 175 carb as i believe that the carb listed has a different part number. I'm also looking for a front pipe, tail pipe, and may be a CDI unit? (I'm sure that the guy at Yamaha said that the ignition timing was different. I guess that it would be pointless sticking with my carb, front and tail pipe as the '83 MX would have been restricted to 12 BHP by the carb and exhaust? Thank you The 125's had a smaller venturi carb, you are looking for a mikuni VM24 for the 175, it will have an ID mark of 2H500 on it I wouldnt bother about the exhaust if you cannot see the restriction in the header pipe I dont think there will be one, the 125's didnt need restricting as the would barely put out more than 12 BHP anyhow, the 175's are only 15 / 16 BHP as standard. Link to comment Share on other sites More sharing options...
PICTURE_MAN36 Posted January 29, 2009 Author Share Posted January 29, 2009 Great I'll have a scout around now. Much Appreciated. I've done nothing with my 125 barrel and head yet so I just need to source a barrel, head, and carb. Cheers Link to comment Share on other sites More sharing options...
Moderator Airhead Posted January 30, 2009 Moderator Share Posted January 30, 2009 and may be a CDI unit? (I'm sure that the guy at Yamaha said that the ignition timing was different. Thank you Your ignition is different..points not CDI, the timing is the same, so stick with what you have because it would cost a fortune to convert. Link to comment Share on other sites More sharing options...
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