overbore kit questions

What do people think of this - http://www.ebay.com/itm/140870811995

Looks like I need to do (at minimum) a hone and new rings, so I was thinking I might order this and some cc's.

Does the kit look any good? Suppose you can't tell from a picture, but if I had it in my hands what could I check to determine that? Pistons all same weight? Dimensional consistency? Anything else?

Assuming I install this, what am I looking at in terms of required tuning and expected performance? I'm guessing my carbs would be a limiting factor, and the pistons could be heavier, so my redline could drop a bit. Seems like myabe I'd want to change the airbox tuning - increase the airbox volume by the same amount as displacement?

 

There was a discussion about these kits before on a thread in modifications. The main question about doing it was whether the crankshaft counterweights would be okay with the larger pistons. A couple of guys were going to do the mod, but haven't heard anything positive or negative about it since. Under 50cc's most likely wouldn't make a difference, but 68cc's might. I don't think anyone knows for sure.

 

Other than increased vibration is the (potential) counterweight issue a concern?

 

The crank counterbalance weights keep the shaft balanced. It can take some changes within reason. Without knowing the crank parameters, adding too much will lead to failure. Like the story of breaking the camels back, one straw too much and the crank will fail. All depends on the crank parameters, it may be able to handle the 68cc's extra. I would definitely put in new crank bearings and shims if it has them, in fact a complete rebuild would be best especially if it has a lot of mileage.

 

Oh, gotahca. Yeah, the stress on the crank journals could be considerable if the piston weight is to much. I'd hope they make them the same weight as the originals, and sent a question to ask that. If they are heavier, I suppose you could try to machine down the skirts, etc.

Would rather avoid a total tear down, though it may be called for. Engine only has 26k miles, but who knows how much neglect? Enough that the rings aren't keeping the oil of of the combustion chamber, anyhow.

 

I have one major problem with this kit.

You will be going from a 65mm bore to 68mm. This will be the same as a 12th OS (3mm). At the 4th OS (1mm) it is standard practice to replace the cylinder sleeves/liners.

Trying to use this kit without replacing the sleeves/liners is asking for trouble as it will leave the sleeves/liners to thin to properly transfer heat to the aluminum casting of the cylinder.

Eventually the sleeves/liners will crack, and distort from the excess heat leaving you will less than a boat anchor once the engine has finished its grenade improvisation.

That kit has been available for well over a year, and as yet has only sold 3. HINT!!!

Ghost

 

I am running the Yoshimura version of this kit. Yes there are performance gains, BUT it does have more vibration than I expected.

No large temp issues so far, and I have an oil temp gauge to monitor it.

Would probably not do the same again.

 

The yoshi kit came with forged pistons. correct?

Oil has minimal contact with the cylinders. IIRC you are monitoring the temp at the pan. Have you looked into checking the temp at the head?

I would be curious what an infrared check would show on the cylinders, and head. I would not be surprised to see a significant difference in temps.

Ghost

 

It's not just the weight of the larger piston, but also the larger surface area creating more downward force. Not to mention the higher compression ratio effects and extra heat on the head.
Hot rodding takes some trial and error sometimes, but the errors are expensive. Hot rodding in a car is somewhat safer with a firewall between you and the motor. The only thing separating you from the motor on a bike is a full gas tank right under your face and against your balls. It pays to examine all angles and tolerances before acting. So find out for sure first and good luck.

 

The Yoshis are cast pistons, I have a Wiseco forged set but I am short some rings for the Wisecos and I ran low on time.

After 3,000klms the plugs show no evidence of overheating, and there has been zero oil usage. I would not say it does not run hotter, but it does not seem to have become a problem. I have concerns because of the climate in Perth being quite hot.

 

Maybe you could use 900 (853cc) jugs, they are 67mm stock so 68mm would be within allowable oversize. Might be wise to use the 900 head, too.

But I'm just guessing here, having never done such a conversion......

 

From all I see here, sounds like advice I'd follow. If I need a re-bore, I'll just go first or second oversize. Hopefully I can get away with honing and new rings.

 

Just a typo correction here.
That would be the 900 (853) jugs.

That would work if the jugs were not as hard to find as an honest politician. Maybe we need to get our friends over in OZ to ship a container full over.

Ghost

 

Yes, XJ900 jugs, good catch (now corrected above).

Honest politicians are eveywhere................just go ask any one that you run across whether they're honest, they'll tell you.

Of course, if one actually came out and said something like "All politicians are liars", then ---- if he is telling the truth (that all politicians are liars), then he too must be lying, so the opposite of his very statement ---- meaning that politicans are NOT liars --- must be true (since everything they say is a lie, including his own statement).

On the other hand, if his statement (that all politicians are liars) is actually a lie, then it shows that the statement IS true , and he is a liar, even though he is, in fact, an "honest" politician.

Which is why you never hear politicians saying that.........

Okay, so much for the philosophical argy-bargy for today. Most of the 853cc (68mm bore) 900's were North American models; all of the '85-up "900" engines (Oz and beyond) were actually 891cc's (i.e. a "real" 900 engine, with the larger BS36 carbs and all), and had a bore of 68.5mm.......

 

the jugs or politicians? muhahaha

*edit: chacal beat me to it, lol

 

I think it would be better to just switch the whole 900 engine. Or even a 1100. Bwaahahahaha

 

Given I was considering picking up a maxim 750 engine, that's not an entirely absurd idea.

What other engines will fit in as drop-in units (matching frame mount and drive shaft mating)? Do they / can they use the same carbs / electricals etc? Cause I doubt those are gonna come with the engine... if I want em, I'd probably be better off shopping for a whole bike!

 

Well the 1100 wouldn't work because it's a chain drive, but the 900 is shaft drive and I believe it will swap into a 750 frame.
Chacal has a write up on engine swaps at XJ4ever. Just select the tab at the top right of this page.

 

NORMAN: But there was no explosion.

MUDD: I lied.

NORMAN: What?

KIRK: He lied. Everything Harry tells you is a lie. Remember that. Everything Harry tells you is a lie.

MUDD: Listen to this carefully, Norman. I am lying.

NORMAN: You say you are lying, but if everything you say is a lie then you are telling the truth, but you cannot tell the truth because everything you say is a lie. You lie. You tell the truth. But you cannot for. Illogical! Illogical! Please explain.

(Smoke comes out of Norman's head.)

NORMAN: You are human. Only humans can explain their behaviour. Please explain.

KIRK: I am not programmed to respond in that area.

(quizzicaldog.jpg)

When did the 1100 magically become chain drive?

 

I guess I lied! Maybe I should run for office. LOL

 

I don't why I was thinking the 1100 was chain drive,must of had it mixed up with some other bike. I'm sure that the 1100 into his frame would require some mods, 900 would most likely swap with ease.