GT550 engine life

[BDcycles]

In terms of mileage, what kind of life expectancy can I expect out of a GT550 engine? In particular the crank and piston/ring life. Mine is a 1974 GT550.

[TheCurtster]

I’m curious also. I can imagine that the answer is a wide spectrum depending on some variables like:

Type of riding.
Temperature of riding.
Time spent at WOT.
Quality of oil used.
Maintenance performance.

I bet an air cooled 2 stroke could be wrecked in as few as a thousand miles, or maybe go as long as 20k before it’s done... maybe more?

Just my thoughts,
Curt

[Vintageman]

typical street riding for younger but mature males. Decent Injection oil (many are). no holes in air filter (not likely) so no debris sucked in.

20,000 miles then need a bottom end inspected and expect servicing needed like Rod big end, bearing, and pin. Top end would have a lot of blow by on pistons by then and should have been refreshed at <15K miles but not too much hp loss. Crank will fail. The early model seams to hole piston easier than later and before this distance, but that to me is simply dirty carbs so poor maintenance if happens under 20000 miles

My experience on couple bikes (both early) and what I have seen for GT550s that have been "parked"

[jabcb]

...The early model seams to hole piston easier than later and before this distance, but that to me is simply dirty carbs so poor maintenance if happens under 20000 miles

My experience on couple bikes (both early) and what I have seen for GT550s that have been "parked"

Suzuki made some changes to the jetting on later GT550s, which may explain the durability improvement.

[Craig380]

Not a 550, but I bought my GT380 in 1998 with 28,000 miles on the clock. It had been cosmetically restored but the PO had not touched the engine, and it had had 6 owners before him.

It was a bit noisy at the top end so I replaced the wrist pin bearings but left the rings and pistons alone. There were wear lips at the top of the center and right cylinders. We found the left jug was 1mm oversize, and the center and right jugs standard bore, but the bike ran fine so I left them alone for a few years.

I eventually found a couple of good used sets of cylinders and pistons at a good price, and after making some careful measurements of piston & cylinder size to get good matches, put a good top end on the bike with new rings and careful chamfering of the port edges. This freshened the bike up nicely.

That top end did 20,000 pretty hard miles, 10,000 of which was with J&R chambers, until I holed a piston (turned out to be a bit of crud partially blocking the main jet, and I was at WFO when it holed). When I lifted the head to inspect the damage, the cylinders were unworn and the ring gaps still well in spec.

The crank was the original, 39 years old and 50,000 miles up. So the answer is, they'll go a long time. I always used a good synthetic 2-stroke oil, the oil pump was set as per the factory (I didn't lean it off - oil is cheap compared to an engine rebuild) and was careful setting the points and timing.

These motors are tough and will withstand a hell of a lot, providing you observe the basics of fuel, timing and oil.

[Vintageman]

If you google there is/was an article that showed how many miles several 70s Jap bikes lasted, based on actual field data. Inspired by that article I have been looking at odometers for example on eBay for mileage they had when "Parked" . I just spent couple minutes looking for that article did not find and did not really care that much.

I had a bookmark to it years ago, but think that is stuck on one of my dead PCs with Hard drive failure. Speak of how long something last: PCs. From what I recall the smaller ones did not last as long and not much past 12K miles (not kilometers),

When I say 20K miles (no proper amount of data to back), I mean if you check things about the crank it may be out of spec... doesn't mean it won't last another 10K for example, But if you invest time and money and want to ride and not worry if you touch redline now and again anything over 20K makes me nervous on these bikes .... always the outlier bike that last longer or shorter.

OH, today you may see one with 5K miles (if very lucky) but due to moisture and oxygen world we live in precision surface rust and once you run will result in early failure. I think that has been more than 50% true for me in New Hampshire

[BDcycles]

Thanks for the feedback. My bike is approaching 17,000 miles. I have no reason to feel the crank is in need of rebuilding but I am interested in putting a fresh top end on the bike and questioned if t was wise to do the crank at the same time. If money weren't an issue then I would just go for it. However, considering the availability/cost of quality components and the labor cost to have the crank rebuilt I would prefer just do the top end. I have multiple street bikes and the 550 isn't my primary ride. I probably will only add about 1,500 miles a year at most to the bike. To the guy with 50,000+ on the 380.... impressive!

[Craig380]

Also worth noting that some of the GT550 crank components are NLA, unlike the 380 (which shares many components with the GT250).

Another story while I remember: I mentioned above that the top end on my 380 when I got it was worn and tired - noticeable wear lips at the top of the cylinders, and the piston rings had lost all their 'spring' - and had gaps bigger than 1mm (which is WELL beyond service limits). However, the compression was still OK (120 to 125psi across all 3 cylinders) and the bike ran well, it would still put 100mph on the clock on a decent stretch of road.

When I replaced the worn top end with a much fresher one (having measured & matched all the piston/cylinder clearances etc) and put new rings in etc, the compression was better (135 psi minimum across all 3 jugs), there was less piston slap and the motor felt a little sharper .... but it wasn't a night & day difference.

These motors are not high-tuned to start with, and a motor with some wear will still perform pretty well providing there's nothing actually broken. My 10 cents is: get the bike set up good (fresh plugs - these bikes LOVE platinum or iridium plugs and they are pretty cheap now, timing & points, carbs sync'd), fresh oil in the gearbox, good injector oil and just ride and enjoy it.

As my dad used to say, if it ain't broke, don't fix it

[Vintageman]

I have no reason to feel the crank is in need of rebuilding but I am interested in putting a fresh top end

Check the conrod side to side rock once top end off. Rust stains on small end rod. That is easy to do.

Check Roller Bearings. I usually do this now and always surprised how much crud is in there so worth it just to clean well. Need to split case to to this though but not that hard.

GT550 crank components are NLA, unlike the 380

.

I am going through a gt380 crank now. Yes, it sure is nice to be able to buy rods direct from Suz 12161-11001, But, I am finding the big end pin 12211-58000 NLA. And, The gt250 has a different number 12211-18001.

I see some NOS now and again but some look to have shelf rust. Some aftermarket in UK?... prefer OEM?... Need to make a decision next few weeks or so.

If you know of anyone with three nice NOS big end pins do tell

[Craig380]

The parts places I can think of trying for big-end pins are Crooks Suzuki, Suzuki Simon, and Robinsons Foundry.

You could also try Pete O'Dell, or other popular ones are http://www.sep-kegworth.co.uk/" onclick="window.open(this.href);return false;, https://www.pjme.co.uk/" onclick="window.open(this.href);return false;, http://www.grampianmotors.co.uk" onclick="window.open(this.href);return false;

I wonder if the 380 crank pin is similar to the one used in other machines with 54mm stroke like the GT250X7, the RG250 or even the RG500? The part number will probably be different but the dimensions could be the same.

[Vintageman]

Thanks

I'll try them

As far as

The part number will probably be different but the dimensions could be the same.

I have extra parts cranks of each (together as an assembly gt380) My first guess to keep the triple from being too wide of engine the crank weights were thinner so pin would be shorter that gt250. Was just a guess maybe. I suppose I can tell that by measuring the cranks I have assembled. THis old stuff is packed away so maybe couple days I'll get a chance to check and reply

Merry Christmas

[Vintageman]

The crank dimension are here ->

https://www.oldjapanesebikes.com/mraxl_ ... ins/page-2" onclick="window.open(this.href);return false; and seeSpecification #7

GT380 looks shorter than gt250

[jabcb]

Interesting. I didn't know that there was an early & late style for the GT750 crank.

[rngdng]

Interesting. I didn't know that there was an early & late style for the GT750 crank.

The rods were different. The early ones used different pistons also. In 74, they changed the rods to the slotted-top style, and the pistons were modified to include the big thrust washers if I remember correctly.

[jabcb]

Did Suzuki make changes to the crankcase & cylinders to accommodate the new crank?
Can you, for example, use an early crank with a later crankcase?