Xj750 to 900 parts fitment information?

So I figured id make a new thread instead of cluttering up the drag bike.

So can you make a 750 a 900? The engine is already torn apart so i could frankinstin soemthing as the drag motor is kind of already anyways. Im not interested in the 853cc version but the 891cc version.

My engine already has 1983 xj900 31a rods in it with a 68.5mm highcompression piston.

Can you get the 900xj valves to fit by sending it to a machine swap? If not can i use a 900 head on a 750? I know the cams will be different. Are the cranks as well?

Id have to find new pistons as the valve reliefs i dont believe will accommodate the 36/30mm valves

Whats the maximum 750 bore? 68.5 is the 891 dimensions.

Or if anyone has a 891 xj900 engine they want to get rid of let me know. Im having trouble sourcing one stateside.

Trying to make the heart of the engine as stout as i can within reason. As it is, ill be sourcing a transmission from the 900 for the 750 case along with the swingarm and shaft (i know its a bit longer). Im just going to put the strongest stuff i can find to hold up to drag hits over and over

Thomas

 

Sure why not we had somebody turn a 650 and to an 883.
Why not find out what motor you actually have by using the short vin?
A 900 motor will fit in a 750 Frame. On the cylinders between exhaust pipe 2 and 3 you'll see the size of the motor stamped .
Your motor does not have 900 crank covers but the 650 and 750 crank covers will fit the 900 motor.
Why look for a 900 motor when you may already have one?
And there was no reason to start a new thread. It's usually easier to keep all the posts in one thread

 

I did use your link and posted my findings in the drag thread yesterday i believe.

Why an actual 900 when i already have the displacement of a 900? I have a 750 built with the bore and stroke of a 900 yes. But its not as strong as an actual 900

Here is a couple things i dont have being a 750
Bigger valves
6 cam towers per cam instead of 4.
The transmission is stronger.

Thats the purpose of this thread, to figure out if i can adapt what i have to make it that or get the correct engine from the get go

 

Bigger valves? Does anyone know how much bigger the 900s valves are?
Stronger gearbox/gears. Is this just about material or treatment? Does anyone know for real.
Cam towers - don't see any real gains here, unless the 750 cam bearings had issues?

Oh, if indeed you do find that you haveto use your fitted pistons, you can have them recut.

 

next step is to look at the jugs and confirm it is a 750 jug that has been turned into a 900. stamped ito jugs

 

750

900

 

@xj550 Went over to the machine shop and confirmed this morning. 5g2 is on the jugs and head. Confirmed the size of the valves as well as being the 750 valves.

@Minimulty as xj550 has posted. 750 has 33mm Intake valves and 28mm exhaust while the 900 has 36/33
~ive only read in one place so far that the turbo 650 and 900 transmissions are stronger.
~the benefit of the cam towers is just additional strenght when im making pass after pass going to redline or higher depending on what i do with the valvetrain
~the worry i have about the valve reliefs is there isnt much meat left before hitting ringland/ring groove area

When building a race engine not designed to be the fasted engine at the track but something you want to last years, you should invest/put time into things that will strengthen the engine. Just my opinion

Thomas

 

Just try to keep in mind that this is a drag race engine guys, even if the xj's werent really drag/supersport bikes

 

Nothing wrong with trying to beef up a weakness, or adding valve area to get more power. I'd be looking for a low miles 900 engine, cheap enough over here. If you're going to explore the top end of the rev counter I would suggest gas flowing the head diy, but carefully.

 

where did you find 5g2 stamped/cast into head and jugs?

 

Xj550 I love going back and forth with you but can we keep posts related to the questions asked please. I started a new thread so the information requested could help someone else out as I haven't been able to find the answers from the searching I did.

 

it is called an exchange of information. you posted about numbers and I asked where they were found. nothing wrong with a little quid pro quo.
we help you with answers , you will be asked questions.

my understanding about the 891 motor is it is the one produced for the rest of the world and not the US. we got the 853 in 1983 then in 84 the 891 was produced.
US did not get any 84 900s.

 

I wonder why they made an 853 then an 891?
What was wrong with the early engine?

 

Yea that's why I don't have much faith in finding a 891 here. I'm sure it can be done, but a bit rare I'm sure

 

Nothing wrong with the earlier engine. The "more power" game was on in full force during the 80's, and every tiny increase in HP sold more bikes.

I'm somewhat thankful that the 'bold new colors/graphics" game took over in the 90's. There were fewer changes to parts each year.

 

Alright gents, i have to set a deadline just to keep stuff pushing for the bike. If no one comes forward with the answers the the original questions by the end of the week. Ill take one for the team and start ordering stuff from the uk. Starting with the head, cams and tranny. Id buy the jugs as well, but if i have a chance at making my pistons fit, ill need to get the 853 jugs and have them bored to my pistons. Followed by the 891 crank, as they dont cost much and im down to spend a couple bucks to see if the two cranks are different for the xj community.

 

Let me correct my information as I was looking at the 900 camshafts on ebay noticed this.

The 750 head has 3 camshaft journals per camshaft not 4
The 900 head has 4 camshaft journals per camshaft not 6

The extra towers I was pointing out arent journals at all, just a support for the valve cover bolts to go into.

Thomas

 

This might be risky; the 891 pistons are 1.50mm larger than the 853 pistons; that's more than the allowable oversize for boring cylinder jugs...........

 

More than allowable or more than made available? I would guess given how well these things last there would be little call for a plus 60 thou piston. The important dimension would be the thickness of the cast liner below the casting.
It doesn't sound not doable, but then I've not eyeballed one.

 

When the maximum OEM piston size is used and then it needs to be replaced the cylinders get resleved.
I would think going to high-compression motor infants sleep walls could lead to fractures.
Occasionally I hear talk about 883 big bore kits for the 750 recently saw somebody offering one on Facebook.
Possibly in the racing world we can constantly rebuild change cylinders making runs then rebuilding cylinders on a regular basis would be a reasonable thing but not something someone would want to do for a street bike

 

Another thought is that if Yamaha could have gotten away with thinner cylinder sleeves to start with they would have done that save themselves some money I'm sure they're engineers have reasons for this current stock sleeve thickness in the maximum replaceable piston sizes. Anything can be done it's just how long it's going to last. Sometimes I do like to trust Yamahas engineers were correct and what they did.

 

https://www.xjbikes.com/forums/threads/650-820cc-conversion.128059/

This link above is about a recent number what time does 650 into 820 cc

He seems to have the skillset to do such things would like to see him weigh in on this conversation he has many YouTube videos.

 

If you take that to its logical conclusion they'd still be selling the xj....

However - what your saying is not wrong - but really, what has it to do with the op's question? He isn't Yamaha, doesn't have a 891, and isn't going to be selling hundreds of thousands of them. In the real world you would simply measure up the wall thickness, subtract 30 thou and make the judgement. Pontificating about the skills of Yamaha engineers of yesteryear doesn't really help?

 

yess he already has some sort of big bore kit in the 750 jugs. which is ok as it is his. he wants to drag all day up to and past red line. big question is how often he wants to rebuild replace jugs while persuing the fastest run. his motor/trans is already beat to shi$ . well he find an 891 in the US I doubt it, he will have to find someone willing to ship one from europe good luck with that and very expensive.
he would be better off getting a US 900 motor and expanding on that. with his wish of stronger parts. have some pistons made for over size.


I have found 2 sources for big bore kits oddly cheap and one guy waiting for his dad to come up with a price on his nos bigbore kit and cam shafts on FB.
I get the desire to chase the lowest 1/4 mile, but quite frankly I do not think the op has the skill set to design/upgrade maintain a machine like that .
his posts infer this.

maybe i read to much into his post

 

@XJ550H this bike wasnt/isnt being built to pursue the fastest run. Its bracket racing, consistency over speed. My goal as originally stated is for it to be as strong as it can so it can repeat the years of racing its already done. Not only that but this is a backup/training bike, better believe i want it as strong as possible when my gal or nephew decide to race. If i can achieve better flow while adding the strength of the 900 head(or bigger valves with the 750 head), great. Will it actually be revved passed stock redline, that will depend on the valvetrain and also if there is any power further up the rev range when it gets dynoed. I appreciate your faith btw. When it comes down to my skillset, my main 2009 Buell 1125r drag bike is way more complicated the any xj and ive raced/worked/maintained it for years.

When it comes to the juggs, id have to call the sellers of each the 853 and 891 juggs for measurements to see if they would even fit in the 750 case. Then determine which has the best sleeve choice for the pistons i have. Id love to measure the set i have now, but unfortunately its still at the machine shop

 

Yes, you need to start measuring the jugs - personally I'd be surprised if they fit the 750 casings, otherwise they would have sufficient "meat" to take the 891 pistons - plus some overbore allowance (thats assuming Yamaha ever made oversize 891 pistons)...
Let us all know?

 

I have a Similar idea, but I'm not sure it will Work. I want to Bore My 750 Maxim to 67mm and run 900 Piston's with the 56.4mm stroke. If My calculations are right that should be 795.39cc's "800". But instead of installing large Valves in the 750 Head it appears the 900 Head may bolt right in. I could be wrong about that though, if anyone knows better please let Me know

 

Also does anyone know where I can get a set of 67mm High Compression Piston's? I've found a few Candidate's I think will Work, but I can't find anything designed for the XJ. I've heard there used to be Kit's, but their hard to find now.

 

This is my number 1 Contender so far. I think it's for an old Honda 3 Wheeler. I can't see what would stop Me from Running 4 of them in an XJ, but I will admit I'm in a little over My Head with this Project. It's not really a Project, just an idea at this stage...