XT 400 Expansion

[Motu]

I've worked out that using the 600 top end will give me a 7:1 compression ratio...not really the hot set up for the 21st century,but I could run any flamable fluid no worries.The 400 top end will give me an 11:1 ratio,a bit borderline,but I think it could take it.But I can relieve the head a bit and bring it down to say 10:1,the only down side is the small valves,but I think we can still flow enough for 7000rpm.

My engine reconditioner is not too keen on the sleeve swap,he reckons it would be cheaper to weld the combustion chamber to get the compression up and get the bigger valves than swap sleeves.Apparently it's a shit of a job machining the barrel to the bigger size,putting the step in the top is the hardest part he says.But I want the stronger castings and better stud spacings of the later top end,so we are doing it my way....next we gotta hope the barrel casting is the same and we have enough meat for the rebore....

[MacD]

How's it all going? I used to own an earlier kick-start only SRX600 (608cc version) and now have an electric-start monoshock SRX400. Reading up on the differences between the XT and SRX versions of the engines I was surprised that Yamaha bothered to make as many changes as they did. Maybe one day I'll look at putting an XT600E engine in the SRX? Apart from the rear engine mount difference, the driveshaft sprocket supposedly has a different offset on the XT engine.

[Motu]

Still statis quo - I had planned this over winter and I'd have 2 bikes for summer,but now the DT230 is my road bike....and I'm kinda enjoying that,but it was supposed to be my off road bike.Oh well,I just go with the flow and enjoy what I've got,every bike is a good ride I reckon.

I'm not pushing my engine man,we have a symbiotic relationship - I give him work,and he keeps my work on track.If I lean on him he might get a bit riled.We have a Jag thing going at the moment - I had a 4.2 lying around my place for a year,he's had it for a couple of years,and now it's all done....and the cars here.A MkII,and as well as taking the 3.4 out and fitting the 4.2,we are converting it to left hand drive for export to LA,with an electric rack and pinion.A freaking nightmare I don't need with a month to Xmas.There are still little things to do to the 4.2 now we have both out....when this is finished I'm going to demand my bits be done.

[Motu]

I got the barrel back a week or so before Xmas,and it's turned out lookin good.We were a bit worried if the casting didn't have the same wall thickness to take the bigger sleeve,but I was sure they were the same casting.So now I have to strip the bottom end so the crankcase mouth can be machined out for the bigger sleeve.I have relieved the head a bit to unmask the valves,I'll do a trial assembly and measure compression volume with a burret and work my way down to say 10:1.Still a long way to go.

[Motu]

One step forward,another back....I just feel I'm going nowhere.

I stripped the 400 cases and was pleased to see everything in top order there,the bearings are perfect and not a mark on any of the gears,makes a change for these motors.But it's all 600 internals with a 400 top end,so not suprising,even at 60,000km.

So the cases have been machined and I was getting ready for a trial assembly when I find out the gudgeon pins are a differnt diameter!! Ahhh! How the hell did I miss that? Both pistons have been sitting side by side on my desk for months,but I didn't notice that the 400 is 20mm,the 97mm piston 22mm,the pin is also shorter.There are no standard bushes that size,so I'll have to get some made,plus spacers on the ends,maybe make the bushes flanged to take them out to the circlip groove.I don't think my engine man will be too happy with fussy work like that,I'll get a mate to make them....this guy was around a few weeks ago and said why didn't I stroke the old flywheels...just weld up the crankpin hole and redrill further out,he does it to Harleys enough.Well too bloody late,we are on a different plan!

I've also just weighed the pistons....

87mm piston - 337g

97mm ProX - 431g

97mm WISECO - 331g

So the piston I am fitting will be 100g heavier than stock,but I don't think it will make that much difference,seeing as the WISECO is 100g lighter than a stock 600cc piston and used as a replacement.The rod eye is smaller in dia,but with the shorter stroke piston speeds will be lower and piston reversal loads less.I've been weighing a few things and a Panasonic D cell battery is 100g,hmmmm,pity I couldn't get a WISECO piston as it's almost the exact weight of the 87mm 400 piston.

[cheese]

that is some big diference, but i think you'll be fine with an extra 100g in the piston.

are you getting sick of that bike yet? sounds like a bit of a mission! need to put some pictures up.

[Motu]

I got the piston back tonight,piston bushed to 20mm,with a couple of alloy spacers out to the circlip groove,a nice job.Cost me a WoF check (he failed,heh,heh) and a couple of BSA sausage mufflers he spotted under a bench.We've known each other for over 35 years,and that's how deals were done in those days - good to see we haven't lost the skills....

I popped the crank in the cases and did a quick dummy up to see how it looked - it doesn't look short stroke at all,I see inside all sorts of motors and this 97mm x 67mm looks quite normal.I was worried about deck height/piston protrusion,and it sits 0.6mm above the barrel,and the used head gasket is 0.9mm,so pretty tight squish clearance,and with the small combustion chamber more squish area than normal.I might do a plastcene check .

So,progess getting made,now I'll have to start ordering parts,gaskets,timing chains etc.

[cheese]

get some pictures of this beast in progress for us!

[Motu]

Dummied up today and playing with a burrett I get 10.36:1...sounds perfect to me.

[Motu]

Did a double check on the compression ratio,I've had 2 at 10.37:1 and one 10.36:1,so 10.37:1 it is.My squish is 0.75mm,about 30 thou which is perfect.There is a lot of squish on this motor with a 400 head on the 600 bore,so need as little as possible to control detonation,without getting into clearance problems...fingers crossed.With a little more cleaning up on the head compression may come down to around 10.25:1,about where I want it.

Now I just have to accumulate gaskets,seals and cam chain - all seperate items on a Yamaha,I hope to have some luck on the internet.

[Motu]

Oh boy,my head is spinning! A few developments today...

I ordered a gasket set the first week of march from Coyote Motorsports,they plucked my money on the 8th,and I have had no contact with them despite several emails,apart from ''Have you got it yet?'' So I have given up and started to put it together using gaskets from Red Baron...and as I expected it's been a bloody hassle.Today I get an email from Coyote saying DHL have been repeatedly trying to deliver it,if I don't pick it up it will be returned to the US with no refund.So I ring DHL to track it - they find it has been addressed to Wellington! Um,no Mt Roskill there! So I should see the gaskets tomorrow.They had an issue with the ZIP code...but we don't have them,I gave them my area code - the new ZIP code system should prevent such hic ups in the future.

A couple of months ago a guy came in looking for a fuse,and said ''What are you doing with the XT motor?' Ah,someone who can tell and XT motor lying in parts,must know them inside out.It appears he has the same problems as me - in reverse! He has an '86 Tenerie with the dreaded 5th gear problem,so picked up a '94 bottom end and used it's gearbox.But he can't use any of the other parts,as it's electric start...and he has the crank! So we do a deal,I get all his '94 parts and I give him my Tenerie tank,he brings in the cases today,and I give him my SRX crank in exchange for a damaged late model head.The con rod is not in good nick (the reason for the swaps is everything is useless to each of us) But I think I can find the right people to restore the rod and give me an 84mm crank.Next interesting thing is the '94 cases have the updated external oil feed to the gearbox,and a higher capacity oil pump...now that'd be nice.

So now what do I do? Fart around trying to make it a 600cc,or be impatient and slap the 500cc together.I think I'll put it together as the 500cc,as I'm kinda interested to see how it goes,then I'll have time to work on the 600cc crank and cyl head,then pop everything into the '94 cases at as later date...this thing has been apart since last year,I want to ride it again!

[NordieBoy]

Do it.
Ride it.

Now.

[XTC]

So is it gonna be ready for clint's ride or not? Even if you have to put the DT plate on it.

[Motu]

So,the cases are together,piston and barrel on and assembling the primary side.Do I fit the SRX kickstart while I'm putting it together? I guess it makes sense to do it now.I fitted the higher capacity 3AJ oil pump from the '94 too,that'll give more oil on the output side.97x67 doesn't look too bad in the flesh,pretty normal really.

It took 11 days to get the gasket set out of DHL - it was addressed correctly from Coyote....this was a DHL stuff up from the start.

[cheese]

4 weeks and I still don't have my pipe from America............