bottom rings on GT750 with 3 mm cylinder riser

[pull the wire]

I have a GT750 I'm building with K model Ocelot ported cylinders and a 2mm cylinder spacer added. The assembled stack height of the spacer and 1 gasket on each side of the spacer is 3mm. My plan is/was to remove 3 mm off the top cylinder to compensate for the 3mm spacer stack height. I did a mock assembly of the cylinder, spacer, base gaskets, and pistons with rings installed on the rods. When the pistons are at bottom dead center, the entire width of the bottom ring and ring end gap is visible/exposed below the top edge of the intake port. I do not want the end gap exposed at all so now I'm a little screwed.

Is it gonna be acceptable to cut 1 mm off the bottom of the cylinder and then remove 2mm off the top of the cylinders? I'm positive that will cure my problem short of purchasing a thinner spacer or leave the spacer out all together. I don't mind removing material off the bottom cause the bottom of the cylinder is a little rough anyway. I know I'll lose 1 mm of the port timing that I was planning on but I'm sure she will still run pretty dang good.

Any advice would be appreciated and thank you all in advance.

[tz375]

Hatchet (Kevin H) has a thicker spacer under his block and he removed all but a tiny amount of the bridge and his bike runs 130 MPH. or so at the end of a quarter mile. The trick is to chamfer the inlets properly to allow the rings to compress as the piston rises.

I plan on using a 3mm spacer on mine and for sure the bottom ring is air cooled at BDC. Was that block run with a spacer previously or has it been aggressively ported and doesn't need one? My Street Tracker makes almost 80 HP with stock CV carbs, UNI filters, DYNA S ignition and a very mild port job on the inlets and exhausts plus JEMCo pipes.

How wild is your porting already and what pipes do you plan on using? Or should I ask - what HP are you planning to achieve?

[pull the wire]

The original owner of this 1973 GT750 was the son of the owner of our local Suzuki dealership. The top end was sent to Ocelot for porting and mill the head when it was new. He installed Ocelot chambers and 34mm Mikuni round slides to finish the job. I have not done anything to the ports since I acquired the bike in 1995 other than clean them up a bit. A spacer has never been used. This set up always ran really good and was very dependable.

I was willing to try raising the port timing just to see if I could safely get a few more HP out of it. My main concern is the bottom ring end gap exposed. It scares me to have the end gap not supported while exposed to the open intake port even for the brief time it is going to spend at BDC.

So I guess the Ocelot porting is on the wild side and I don't need the spacer to complicate things???? I may just forget about using the spacer for this build and run what I have. As I mentioned, this bike always ran really good!

[ja-moo]

Where exactly is the ring gap in the port?

[tz375]

The roof of a stock GT750 is typically around 78.5mm below the top deck. Raise that by say 3.5mm and it's now at 75.0mm below TDC . the lower edge of the bottom ring is 12mm below the crown, so at BDC it should be 64.0 (stroke) plus 12mm i.e. 76mm below the top of the block and will start to be exposed in the intake port.

Arithmetically that sounds right but I am seeing the whole bottom ring exposed in my barrels with a spacer, so on those barrels the port roof must be higher. A common tuning trick is to raise the roof to 75mm to be level with the bottom of the piston skirt and slipping a spacer in makes for a lot of exposure.

The ring peg is set somewhat close to the center of one port winder and that's perfectly acceptable. If that was a single 48mm wide port without a bridge or dimple, the ring end would spring into the port and snag. Where it is, I don't see as a problem. JA-Moo can share what happens in a typical Kawasaki triple port

What it all comes down to is the current port shapes and sizes and how close they are to being optimal for the way you ride and what your objective is. If you are targeting over 100HP, a spacer is a must unless you weld up the cooling passages above the exhaust ports. And if you want over 120, the transfers are in for a serious rework. If you want over 200, contact Brett in Australia and see if he's sell you one of his 1000cc 6reed block motors.

If you want to send me the port dimensions by PM I'll run them through MOTA and may have some ideas.

[water cooled]

Unless your pistons were modified and the ring pins relocated for some reason, the lower piston ring gap will not be exposed to the intake port window. It should fall between the transfer window and intake.

The BLACK marks are the top ring pin locations and the RED are the lower ring locations.

[pull the wire]

when looking into the intake port for the RH cylinder for example, the ring end gap is visible in the upper left corner of the intake port. I'm using 1.5mm over Wiseco pistons that are new out of the box. The intake port bridge is in place and is 3 mm wide.

TZ, I will get the port specs to you. Would you like it in crankshaft degrees or distance from the top of the cylinder??

[tz375]

Kevin, On my left and center pistons, the lower ring peg is close to the center of the right port which is a safe place for it to be. The "best" place would arguably against the cylinder wall between ports and well supported, but on mine at least, that's not where the peg is. That's one of many issues with adding boost ports that need to be centered on the inlet port to support the ring but also need to be opposite the exhaust port which is not the same place at all

PTW, Distance from the top please - as close as you can get them - or a port map would work too. I can work in chord width or Arc width as long as I know which way you measured.

[jeff kushner]

I think that you're going to be ok PTW....that's pretty minimal and with the bridge intact, you've got support pretty close. Be sure to camfer the top of the intake ports for a nice gradual guide to the cylinder wall and use a mag glass to see it clearly. I don't know what you have in the way of porting tools but with the proper tools and some careful hand work, you'll be fine. I've had worse rev to nearly 10 grand w/o problems because they were properly prepared. You can also order base gaskets of anywhere from .12 up so if you really don't want to live with this, you can order a couple of very thin gaskets and effectively raise the ring up to 1.5mm. Lastly, you have the option of dropping the bottom ring altogether. A little more power due to less pumping loss but there's the trade off with longevity.


jeff

[Suzukidave]

Hummmm .. just wondering with the intake port that wide did it expose the transfer opening cut into the piston on its way up ?

[Suzukidave]

That's one of many issues with adding boost ports that need to be centered on the inlet port to support the ring but also need to be opposite the exhaust port

I figured a boost port of @ 17mm smack dab between those 2 ring end gaps that are 20mm apart and that will point at the exhaust .

[tz375]

Dave, I do have one set of barrels ported by an OP where teh intakes were opened up so much that they do not seal on a stock piston.

Removing the lower ring supposedly makes more power about 8,000 and less below that, but I cannot for the life of me remember what motor that was tested on. If that applies to a GT750, it's a problem though because they never see those sort of revs on the street.

Your boost port is fine. I was trying a pair of 20mm ports and the ring pegs are in the wrong place for that. I'd need to move the pegs and so far I haven't got around to it. I built a couple of other bikes instead..... I should go back to that motor again and see where I got to.

[Suzukidave]

Dave, I do have one set of barrels ported by an OP where teh intakes were opened up so much that they do not seal on a stock piston.

Sounds like the perfect cylinder to add reeds too

[ja-moo]

It wouldn't bother me in the least to run it with the gap there, the ring is supported well on both sides.

[pull the wire]

Thanks for your advice folks. I'm going to run the spacer and go for it.

BTW, I acquired a 1985 RG500 Gamma frame only if anyone is interested. My GT750 frame weighed about 50lbs. The RG frame weighs 19 pounds. WOW

I know what your thinking and the answer is the GT750 engine is way too big for that frame without hacking it to pieces.

All I have is a bill of sale and I'm positive it is not stolen. Long story short is I work with the guy (Terry) who purchased the complete motorcycle years ago and he stripped it and sold everything on ebay. By stupid chance, I bought it from the ebay purchaser and he told me he bought it from my co worker Terry.