Big bore short stroke modification

Alright guys relatively new to the world of xjs and first impressions these things are high rpm screamers!!! I'm absolutely digging this bike but would like to get a bit more out of the bike.
So here's my thoughts
I have a 1980 xj650 maxim has anyone machined a 900 jug down to keep the 650 stroke? The reason this crossed my mind is because having the shorter stroke can achieve higher rpms and regearing these bikes seem quite impossible.
Xj900 head and cylinder
I found a site that regrinds cams
And id like to try the Murray carb setup

Any suggestions and input is appreciated

 

Putting bigger pistons on an engine means you keep the rpm capability but make more power - essentially correct.
But, to put a damper on this, there are negatives.
Bigger pistons are heavier, so knocking some of those rpms off.
Bigger pistons need more air to make more power, but can you get more air through the 650 head? Could you use the 700 or 900 head (on the assumption that both have bigger ports than the 650).
If the bigger heads flow better, can the required comp (CR)ratio be achieved?
Assuming the mechanicals all work, then you need to fit the right carbs and correctly jet.
So yes is the answer, if you have the workshop and skills, but its a lot of work

 

I work in a small patternshop they will let me use machinery. Do you know If the compression height of the 650 and 900 pistons are the same? Meaning center of wrist pin to top of piston.
Is the bolt pattern the same for both jugs.
Wrist pins the same size?
Will timing chain line up?
Might look into aftermarket replacement pistons to cut down on weight. Vibration from the weight was something I thought about as well but figured I could weigh parts if I decided to tackle it.
In terms of air flow I figured the 900 head should do it plenty considering it will be a smaller displacement.
I'm serious about trying it.
Side note... should have a 98 royal star swing arm in the next couple days saw it was longer and no one had done it yet will update when it comes

 

Some stuff I can answer, @Simmy would be your best bet:-
Do you know If the compression height of the 650 and 900 pistons are the same? Meaning center of wrist pin to top of piston. No it is not, pretty sure.
Is the bolt pattern the same for both jugs. Yes (from @Simmy)
Wrist pins the same size? Yes, pretty sure but not certain.
Will timing chain line up? Apparently yes (from @Simmy)
Might look into aftermarket replacement pistons to cut down on weight. Vibration from the weight was something I thought about as well but figured I could weigh parts if I decided to tackle it. If they’re all the same all you’ll be doing is stressing the bottom end more, so get lightest possible?
In terms of air flow I figured the 900 head should do it plenty considering it will be a smaller displacement. Should work, maybe retime the cams to suit the shorter stroke from 900.
I'm serious about trying it. Let us know!
Side note... should have a 98 royal star swing arm in the next couple days saw it was longer and no one had done it yet will update when it comes[/QUOTE]
Some stuff I can answer, @Simmy would be your best bet - see above. At least you’re asking the right questions.

 

I have some answers for you but until you attempt this yourself, you are really in unchartered waters.
As @Minimutly has kindly made reference, you can go to my thread here https://xjbikes.com/forums/threads/simmys-naked-turbo-project.110254/
Although it is quite long and mostly detailing refurbishment of the standard turbo running gear, I have some segments illustrating my side project of making more displacement for a turbo motor.

First up, the pistons.
Have a look at this pic.
It is a 650 turbo piston (63mm) connected to 2 pistons from the 853 (67mm)

The wrist pins are the same diameter but slightly longer in the 900.
I'm not sure what the standard 650 piston looks like but you can see the piston profile of the turbo and the 67mm is different.
The only way to really determine the CR is to actually assemble the motor with your chosen parts and physically measure the combustion chamber.
You only need 1 piston installed. Smother the piston & rings with grease and slip the cylinder block over the piston.
Put that piston at TDC. Install the head (and base and head gasket to of coarse).
Now fill the combustion chamber to the spark plug threads with fluid (WD40 is good) from a graduated cylinder (or burette if you're fancy) and determine the volume used.
Comparing that with your bore & stroke will tell you what your CR will be.

Yamaha 1st built the 653 then the 653T, 748, 853, 891 and 695 all using essentially the same cases.
The 900 also has shorter connecting rods so the block is almost the same height.
I believe everything will bolt together but without going through the process above you won't know if you need more compression or less.

Below is the piston from the 891 (68.5mm) I'm trying to make work for my turbo project.
The 68.5 bore with 52.4mm stroke makes a 772.
67mm bore with 52.4 stroke makes a 738.6

 

I just came back from a blast up and down my local expressway. The Turbo is running really strong as a 650 with 9lbs boost. Seems a shame to mess with it but that’s why I have the other motor.

 

Is the combustion chamber in the head shaped differently? Looks like the crown of the pistons are different
Does anyone who where we can get NEW pistons? I can seem to find any specific to the 650 or any of the xjs pre diversion

 

I think you have to assume the combustion chamber shape is different for all the various displacements Yamaha made.
The pic I attached compares a Turbo piston to the 891, I've not had a standard 650 apart yet.
Yamaha dropped the compression in the Turbo from 9.2:1 to 8.2:1. The piston is flat on top.
Combustion chamber on Turbo is listed as 22.7 cc
Combustion chamber on XJ650RJC Seca (YICS) is listed as 24.2 cc
Oddly enough, the lower compression motor has a smaller volume in the head.
I understood you were contemplating using the 900 head, piston and block so the only thing to concern you is the deck height.

here is one but it looks like they no longer offer it.
https://www.cruzinimage.net/2017/08/25/81-84-yamaha-xj750-818cc-bigbore-piston-kits/
Even if you could get it, the kit is made to bolt into a 750.
If you were to install this in a 650 you would need to go through the process I outlined to determine your CR.
And if you're doing that you're better off getting good used Yamaha parts than some shitty stuff from China.

 

Hey @Simmy,
I'm about to order a cylinder online just wanna make sure I get the right stuff.
I want to use the bigger bore of the 2 obviously. So that is 85 to 93 correct?

 

Also what is 31a vs 58l?


Edit..
The 31a is the 67mm bore the 58l is the 68.5. I think
Will a 31a head work on a 58l cylinder?

 

Would it be easier to install a 900 motor and carbs into your 650 same as putting a 396 Chevy into the Nova that came with a 283

 

Yes I'm aware.
My theory is that keeping the short stroke of the 650 will help maintain rpm capability.
I also think there will be less "non mechanical" work in swapping the top end. Meaning sensors wires the little stuff like petcock clearances that people seem to run into when doing the full swap. I'm not sure where the height comes from but it seems to be a common problem.
Another factor to mention is that finding a complete a pre diversion xj900 in operational condition is next to impossible. There's one on ebay rn for 900 dollars plus 300 freight shipping but its the striker 67mm apposed to the 68.5 o the later years. And dropped 12-1500 in one sitting is a bit much for me at the moment. Doing it this way I canspend a couple hundred at a time and it gives me a cool project. That I'm aware of it hasn't been done YET tho simmy is close.
Another plus is that numbers stay matching but I don't really mind that much.
I'll give it an honest try hopefully have the necessary parts and information to start it once snow Flys.
Easy isn't the goal. Possible but improbable? I'll try it

 

If you can hold off for a week or so I will know if this is all going to work out.
Last night I tried fitting just #1 into the block but at 62 I guess my dexterity is not as good anymore.
No bifocals yet but it's time.
I ordered a ring compressor and once that arrives I'll get it together for a trial fit.
Maybe with help I can fit it tomorrow.

Here are the variables;
The connecting rod of the 900 is shorter than the 650T.
The piston of the 900 has a different crown profile
The block of the 900 is 1/2 mm taller.

If the piston crown ends up too high in the cylinder then a shim might be needed to raise the block.
In my case I have no room for a taller motor. You have alluded to tight fit with the petcock.
My case is worse because the Turbo has a bigger petcock. It is already a tight fit.
Alternatively I could carve some out of the head to make more volume.
Just keep in mind I'm striving for around 8:1, you want to be > 9:1.


650T conn rod is on top. 31A (853) on the bottom.


the Turbo rod S/N starts with 5G which I think means it is a modified 750 to begin with.
The 650 conn rod starts with 4H, I don't know if it the same length as the 750.

You have a 1980 650 which means no YICS.
I suppose you could put a non-YICS head on a YICS block, just not the other way around.

 

Yes I'm absolutely ok with waiting and I do plan to put a yics top end on my non yics bottom. Does the sleeve that hangs out of the bottom of the jug fit into the 650 block?
Do you plan on balancing the rotating assembly?
And are you going to use the 900 head? If so I could think that the head is designed to match the 900 piston at least more closely then it would your turbo piston

 

yes it does, good clearance there.

no. I suppose I should compare the weight of the pistons and wrist pins.
I have another Seca with an XJ750-X motor. I installed FZR1000 block and pistons. The pistons were a few grams heavier but no noticeable increase in vibration.
That was over 20 years ago, still running strong.

I do not plan to use the 900 head.
The 900 cams have 4 journal support VS just 3 for the 650/750 cams, that is why the 900 head is larger.
The Turbo has shorter duration cams specific for turbocharging and of course do not fit the 900 head.
But you are correct, I have some work to match my Turbo head to the larger bore.
The Turbo valves, although same size as the standard 650 are of upgraded metallurgy, something you don't get with the normally aspirated heads.

 

You may need to block off the YICS ports on the head. I can't remember if they overhang the non YICS cylinder block enough to cause the ports to be open or not.

 

Anyone have a picture of the port? I'd use yics jug and head

 

They are a series of ports; one per cylinder going from the combustion chamber to the rear of the head where they mate with the four openings in the YICS manifold that's cast into the cylinder block. If you use like=like then yo won't need to bother with blocking anything.

 

@Simmy
Got a post on the fzr build? How'd you run a water pump

 

I suspect you're referring to my 911 Seca.
I started with the XJ750-X motor which already has a water pump.
I do not have a proper build thread for this as I built it before this current forum existed.
Essentially, the motor is the 750 Maxim-X with 75 mm pistons and cylinder block from the 87/88 FZR1000.
The block had to be shortened and the boss for the water cooling connection on the back of the block had to be altered.
I went through the same process of assembling the motor then taking a fluid measurement of the combustion chamber.

 

@Simmy
Hey was wondering if you could take a measurement for me..
The intake looks like this
○ ○ ○ ○
O O O O
○ ○ ○ ○
Sorry idk how to upload pictures. But I'm wanting to do some fab work and was wondering if you could tell me what degree the bolt holes are at. Center of intake being start point and straight up being 90°

 

My attempt at mapping out the bolt holes failed

 

sorry @Phoenix starr but I don't understand what you're on about.
Are you talking about the rubber carb manifold bolt holes?

 

Yessir I thought you might have a head easily accessible

 

OK, you might have to wait until I'm back home next week.
I don't have anything more accurate than a small carpenter's protractor.

I understand you may be using the non-YICS 650 head which has different bolt pattern than the YICS heads.
1 and 2 are not mirror images of 3 and 4 like the YICS heads.
I have heads from the Turbo and the 853.

 

On this same theme, can I use an XJ900 crankshaft in an XJ750 engine? The stroke is slightly longer and connecting rods shorter, but otherwise looks possible since everything else below the barrels seems identical.

 

If you're contemplating just the 900 crank with 750 pistons, the cylinder block will need to be taller.
A spacer would need to be made between the cases and the block.
Will the sleeves be long enough? I don't know.
And then you might need a longer cam chain.

 

I got thinking about the cam chains.
I don't see how extra links could be added without making it way too long.

the caliper above measures 16 mm. This is what 2 more links measure, you can't add just one link.
So if a 750 with the 900 stroke can't be fit together with the OEM cam chain you're SOL.