XT 400 Expansion

[Motu]

My XT 400 is getting a revamp - I thought I might let you follow the pitfalls that I might encounter along the way.The XT400E Artesia is just a small bore XT600E,the same as the SRX400 to the SRX600,exactly the same bike,just downsized for tax purposes in Japan.The 400 is ok for what I need,it can handle the easy 4x4 tracks I use it on,it's actualy great on gravel - being underpowered I can give it heaps and just ride it flat,like an XR200,I don't have to control wheelspin.On the open road it can sit on 100 to 120 all day,but keeping it there can be a pain on hills and in wind,and passing is a very well planned event.With the loss of the XLV750 the bike cries out to be a 600....

Enter White Trash - all hail Jimmy the gigantic,the bringer of gifts to the helpless and needy.When flicking off his SRX600 track bike,he tossed a spare engine my way,it took a bit of getting to me,but the motor arrived and my XS1 left in the same vehicle - us bikers is good people eh?

So,the Yamaha single family...the learning curve begins.XT,TT,SRX,600cc,400cc...all the same,all different.The TT is the off road racer,lighter,more power,no electric start and a nikasil bore,nice if I could get one.XT is the trail bike,with electric start on the XT600E from about 1988?.SRX is the cafe racer,no electric start (ok,so late models have one,I know) and a tacho.Now a funny thing - XT,TT and late SRX are 595cc with a 95mm bore,but the first SRX600 had a 96mm bore and was marked 608cc on the barrel.The 400s have a smaller bore (85 I think) and shorter stroke,everything else appears the same,I think even the carbs are the same,same casting anyway.Apparently the SRX600 has the XT400 gearbox,I haven't checked ratios,but I think the XT400 would have a closer ratio box,so gets used on the street bike.The SRX has a weaker top end,weaker rod,barrel,smaller valves and a different stud spacing on the head,but the barrels will bolt on.Also the swingarm mounts through the crankcase on the XT and has a bigger hole.

What I intend to do is put the SRX600 crank and top end on my 400cc cases,that way I can keep the electric start,fit a kickstart as well and just generaly keep a later model bottom end.The crankcase mouth will need to be bored to take the bigger barrel,no biggie,but I hope(fingers crossed) that we will have a bolt on job.

I have stripped the SRX and found my first problem - a 97mm Wiseco piston - smashed! The crank is in great shape.To go up to a 98mm piston means the compression goes up to 10.2:1,not what I want to put on the weaker SRX rod - but we hone and measure the bore - only a thou wear and no marks at all,so I can keep to 97mm...that's 621cc.I can't get a Wiseco,well I can,but it needs to be imported,so settle for a ProX job.This looks bigger than the cut down forged Wiseco piston,but is the same pattern as original,so should be fine.As to why the Wiseco piston failed? It has been running for some time like that by the wear pattern,and it's the non thrust side that is missing(found some of it) - my guess is that the balance shaft has been incorrectly timed and knocked the skirt,either snapping it off,or cracking it and failing later on,but of course I never thought to check the balance weight timing before I ripped it apart.

The rest of the job will have to wait until I can take the XT400 off the road,either when I get another bike,or wait till winter.Here's a picture of the Wiseco and ProX pistons next to the barrel.

[Bonez]

Looking fwd to more installments.

[Motu]

Looks like I had to wait for winter - I didn't expect to buy a house Xmas Eve and spend 6 mths doing it up and shifting.

So out came the 400 motor and I'm pulling it down,I've got the top end off and got some photos to show you the difference in bore size.Same castings,but the 92 XT has some differences to the old SRX,a different stud spacing on the head and longer head bolts - as I mentioned the later XTs were stronger in the top end.The inside of the 400 is spotlessly clean,which is a good sign,looks much better than the 60,000km it's done,confirming that these things will last forever.

The valves are smaller in the 400,but port sizes the same,carbs the same size,camshaft identical - no wonder it would hardly run with the airbox opened up,they just restricted the air intake at the filter.

I'm stuck getting the flywheel off,it doesn't want to move and I'm leaving it with pressure on while I give it stern looks and mutter threats about heat and hammers - if it doesn't do as I command tomorrow I will have a quiet word with it alone....just the 2 of us and some nasty tools.My main worry is if the cranks will be the same.

[Artifice]

considering the effort they didnt go to to make the carbs different, i doubt they made the casings any different either. sure the 600 crank probably needs another 10mm or so of space and the counterweight will probably be thicker. on second thoughts i rekon they designed the 600 and then just restricted it down for the 400. maybe the 400s counterweight is the same only got some of its guts missing. flatmate did a similar job on his klr600-650. needed to put in a spacer at the bottom of the head to make room for the longer travel. as you have a 600 head i guess its long enough allready. just measure it first ay.

good luck with the build. sounds like a cool project.

[Motu]

The SRX is an 86,the later models had wider stud spacings and you can see an extra bolt hole in the chain tunnel....so the heads won't interchange,but the whole top end will.The balance shaft will be a different weight,that's no worry - but the SRX is kick start,the XT has electric...and a different flywheel with start gear etc,they look the same in the parts book....and I hope in real life too.

[Wellyman]

Good luck Motu. The pistons well the look intresting to say the least. Goo luck with your rebuild and keep us posted. I think my CRF is probaly due for a bit of an overhaul.

[Sensei]

Coul you not put the whole SRX600 into ya XT ?? Did a 640 kit into my
1990 TT600 breath taking power .

[Motu]

I want the original cases and electric start,putting the SRX crank in XT cases is easier than adapting the SRX to electric start.The bigger bore kits have higher compression,not what I want...although I think the 101mm piston is standard comp.

[Motu]

Oh dear,what I was hoping wouldn't happen,did happen - the cranks are different!

I was put off a little as I have the factory XT manual,it shows engine strip and rebuild with photos and exploded drawings,and with the SRX crank in my hands they looked the same - but now I realise they must of been using a non electric start model for the photos,and only putting in electric start stuff when appropriate.Later in the manual under Alt and electric start I see the correct crank,but I didn't look there eh? Wish I had of known,or knew somebody who knew,I could of spent these 6 mths rebuilding the SRX and just done a swap,now I have TWO motors apart! Trouble is with Yamaha you can't buy a gasket set,you have to get each gasket,seal,Oring seperatly,what a pain!

[Motu]

I have a new plan,deep in the shit,the fertility of my brain plumbs new depths.I'm gunna make an XT500!

An XT600 is 95mm x 87mm,an SRX 600 is 96mm x 87mm,an XT400E is 87mm x 67.2mm....I have a 97mm piston.I'm going to use my 67.2mm crank,and just put the 97mm top end on,making a short stroke 97 x 67.2 motor - Mr Skid and I worked it out to 495cc,pretty close huh?

Now,the complications.....the cases will still need machining for the bigger sleeve,so I still have to strip the XT400,can't just slap the top end on.It will lower the compression with the shorter stroke...is this a bad thing? enough to notice?....I will do some measurements.I could put the big sleeve into my 400 barrel,and use the 400 head,same size ports,but smaller valves,but this will raise the compression....by how much? more calc required.A jetting nightmare with carburettors with unavailable jets.

I like the idea,no big cubes is a let down,but a short stroke big bore is an interesting idea....grist for the mill in my brain....

[Sensei]

At least it's only 1 piston Motu

[sels1]

When you first started this thread I owned a XT400, so I watched with interest.....but then I couldnt wait for the next installment so went n bought the F650 Good luck with the project!

[Mr Skid]

I have a new plan,deep in the shit,the fertility of my brain plumbs new depths.I'm gunna make an XT500!

An XT600 is 95mm x 87mm,an SRX 600 is 96mm x 87mm,an XT400E is 87mm x 67.2mm....I have a 97mm piston.I'm going to use my 67.2mm crank,and just put the 97mm top end on,making a short stroke 97 x 67.2 motor - Mr Skid and I worked it out to 495cc,pretty close huh?

Now,the complications.....the cases will still need machining for the bigger sleeve,so I still have to strip the XT400,can't just slap the top end on.It will lower the compression with the shorter stroke...is this a bad thing? enough to notice?....I will do some measurements.I could put the big sleeve into my 400 barrel,and use the 400 head,same size ports,but smaller valves,but this will raise the compression....by how much? more calc required.A jetting nightmare with carburettors with unavailable jets.

I like the idea,no big cubes is a let down,but a short stroke big bore is an interesting idea....grist for the mill in my brain....

I've worked out the formula for maximum piston speed that i was telling you about - should be able to model the piston's travel on a sine wave, and differentiate to work out the redline based on the stroke. That doesn't take into account whether you'd get valve float or something other nasty at high revs.

The XT tacho only goes to 9k rpm though - you might need to fit an Apexi monsta tacho to the Renthals..

As for jetting I reckon it's going to be enough of a mutant to justify dumping the YDIS carbs.. Didn't one of the SRX sites suggest that the carbs from a gpx250 would bolt up happily to the manifolds?

[sAsLEX]

the formula for maximum piston speed that i was telling you about - should be able to model the piston's travel on a sine wave,

blah blah I do Science.....................



I reckon it should be a cosine wave!!!!!!!!!!





But in all soberr(splelt this wauy when sober!!(_ ) it is cool to see peopleple fiddling!

[Mr Skid]

blah blah I do Science.....................



I reckon it should be a cosine wave!!!!!!!!!!





But in all soberr(splelt this wauy when sober!!(_ ) it is cool to see peopleple fiddling!

I'd have far more faith in your mathematics ability than mine (though not right now, from your spelling).
I enjoy maths, I'm just not terribly good at it