Re: '75 suzuki gt250
It has been known to me for some time now that a suzuki t350 top end is a "bolt on" for the gt250. However, trying to find all the right parts has proved to be difficult. The headers are slightly larger so new exhaust had to be found, along with decent cylinders, carbs, heads, and the oil pump. I know of only one other person on the internet who did such a conversion and I had the pleasure of sharing information with him. Even though he sold the bike many years ago, he still has his website up and many pictures to look over:
http://www.street-unique.com/
What I'm attempting to do is provide as much information I know to people who are both interested in and willing to attempt what I've done. I'll start with what I've got. I purchased a complete t350 engine including the oil pump, one exhaust and both carbs. I was able to salvage the cylinders, covers, oil pump, and one carb. I purchased the other carb and exhaust from ebay. The carbs are a large 32mm, 26mm being the 250 standard. I had the cylinders bored out .5mm, turning the total cc’s from 315 to 320cc. Oh, I didn’t tell you? The t350 is actually only 315cc’s.
This is where it gets tricky. I've heard many times that the t350 was simply a punched out version of the 250. In some respects thats true, however you have to look at the iron linings. The t350 has a larger lining allowing it to be bored out to 61mm, 54mm being for the 250. You have to keep in mind that the oil pump off of the t350 has a larger rate of flow, so that will also be needed. The pistons were the more difficult part. It is fairly easy to find standard bore pistons and rings on ebay. However, you need to understand that there are differences between the years. Some had "thick" 2mm rings and others had "Thin" 1.5mm rings. I bought a complete lot of pistons and rings assuming everything fit, but that was not the case. I also would like to describe the differences between t350 and gt550 pistons. The rings fit on either piston. However, the pistons themselves are different, but both can work if you do a little machining.
I had in my possession both t350 and gt550 pistons. The t350 pistons look to be the correct dimensions compared to the stock 250 pistons. However, I only had one of them, and two gt550 pistons. Gt550 pistons have the piston pin hole placed lower on the piston, resulting in the piston protruding from the cylinder about 4.5mm. So, a spacer needs to be made to bump the cylinder up to the correct height. When this is done, the intake and transfer ports are slightly covered by the piston, so you need to remove about 6.5mm off the bottom of the piston, and dremel out the cutouts on the side of the piston up to the bottom of the piston pin hole. I have a crude ms paint pic showing what I mean. Note, that is not the gt550 piston, but just a pic I had at my disposal. With the piston cut to allow the correct stock port durations, the hardest part is over.
The carbs are a larger version of the mikuni vm26, bolted on with the flange mount. They are also very similar on the inside, which was a relief when cleaning out the many years of old gas residue and dirt. I’ll be able to find out if the gt250 throttle cable is functional with the vm32 carbs in a few days when everything goes together.
The heads are similar in design to the gt250. No sign of a squish band, but a smooth hemispherical shape with the spark plug close to the piston. Not the best design, but I can deal with that later. The exhaust is similar in design to the gt250, other than the headers being about 2mm larger in diameter. The rest of the exhaust is worthless to me, I only needed the headers to be welded onto my already made expansion pipes.
I’m in the process of doing this swap on my 1975 gt250, and should be ready to start this weekend when my correct rings come in. I’ll have the pics of the actual modified pistons as well. Hopefully all this work will pay off. Looking at old road tests, the gt250 put out 32hp and 23 ft/lbs of torque. The t350 put out 39hp and 29 ft/lbs of torque. What can be easier to increase your torque by 26% and the hp by 22%?
