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Discussion Starter #61 (Edited)
This XT case has an extra large case mouth to accommodate the original "Nikasil" cylinder. So, I decided to have LA sleeve make a cylinder to fit.

The sleeve has a 74mm bore (of course), and a 78mm outside diameter, including the lower skirt. It turns out that a full custom sleeve is quite a bit more expensive than picking something more "off the shelf," like a Raptor 250 sleeve.

I didn't get off quite as cheaply as VanillaGorilla did:

Sleeve - 1.jpg

When I was planning this engine, I did a compression ratio calculation, and it came up quite a bit lower than I was hoping for... somewhere around 8.5:1 I think. So, I had them machine .025" from the base of the cylinder before they installed the sleeve.

Sleeve - 2.jpg

Sleeve - 3.jpg

I figure I can always shim this back up with gaskets if I need to. With rough calculations, I think this will get me up over 9:1, which should make for a nice running 250.

I'll update and post my compression ratio calculations when I get the cylinder and piston in place.
 

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Im in the same spot as you and need some help to move forward! I did my calc today and came up with 8.3:1! Im planning to have the top of the cylinder decked to 0.0 like you did except on the top (currently .022" deck height). Im suspect of the head that I have having a lot of volume. What head gasket are you planning to use??? I can't find one that is the right bore. the one Sebastian recommended isn't for a 74mm bore and thick as hell! I'm thinking to call cometic and see what they have to say. their thinnest offering is .027 near as I can tell. Im putting some thought to stroking the crank to make the deck height zero. that will give it more swept volume and help.....

This XT case has an extra large case mouth to accommodate the original "Nikasil" cylinder. So, I decided to have LA sleeve make a cylinder to fit.

The sleeve has a 74mm bore (of course), and a 78mm outside diameter, including the lower skirt. It turns out that a full custom sleeve is quite a bit more expensive than picking something more "off the shelf," like an XT250 sleeve.

I didn't get off quite as cheaply as VanillaGorilla did:

View attachment 200298

When I was planning this engine, I did a compression ratio calculation, and it came up quite a bit lower than I was hoping for... somewhere around 8.5:1 I think. So, I had them machine .025" from the base of the cylinder before they installed the sleeve.

View attachment 200300

View attachment 200302

I figure I can always shim this back up with gaskets if I need to. With rough calculations, I think this will get me up over 9:1, which should make for a nice running 250.

I'll update and post my compression ratio calculations when I get the cylinder and piston in place.
 
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Discussion Starter #63 (Edited)
Wow, you're right!

My "no squish" calculation comes out to 8.3:1, and my "estimated squish" calculation comes out to 8.8:1.

Here are my numbers (assume cylinder bore area of 43 sq cm):

no squish
deck depth .025 cm
head gasket thickness .15 cm

sweep vol. 249.0 cc
head vol. 23.5 cc
piston dish vol. 3.0 cc
deck vol. 1.1 cc
head gasket vol. 6.5 cc

BDC vol 283.1 cc
TDC vol 34.1 cc

Comp. Ratio 8.3 : 1

estimated squish
deck depth 0.0 cm
head gasket thickness .13 cm

sweep vol. 249.0 cc
head vol. 23.5 cc
piston dish vol. 3.0 cc
deck vol. 0.0 cc
head gasket vol. 5.6 cc

BDC vol 281.1 cc
TDC vol 32.1 cc

Comp. Ratio 8.8 : 1

If we can find different head gaskets, .8mm thick should yield a 9.3:1 ratio, and .5mm thick should yield a 9.7:1 ratio.

Deck - 1.jpg
 

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Discussion Starter #64 (Edited)
About that head gasket:

I've also got the Weisco gasket set that Sebastian recommended, and the head gasket bore measures 73.8mm.

If there were some deck space, then I don't think it would really matter if the gasket had a slightly smaller bore... but with zero (or less) deck height, it becomes very important!

I'd be interested in finding a thinner (.5mm-.8mm) 74mm bore gasket as well. In fact, I'd probably be interested in more than one. ;)

Deck - 2.jpg
 

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I have been warned by more than one party about having a gasket small than the bore. The word is that the edge of the gasket can get hot and cause detonation!

that surprises me that your head volume came out so much lower than mine. I did it a few times and got 24.7 every time! Im going to try another calculator to see if they all match. Im also a little worried about valve clearance since I got the big cam as well. last thing I need is to break the valves off on start up.

I went and bought shorter bolts and copper washers so I could put the base gasket in and torque the cylinder down to get the deck height.
Untitled by dolphaxel, on Flickr
 

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Discussion Starter #66
I have been warned by more than one party about having a gasket small than the bore. The word is that the edge of the gasket can get hot and cause detonation!
Good point!

That surprises me that your head volume came out so much lower than mine. I did it a few times and got 24.7 every time!
That was with a TTR225 head that I don't have anymore. For this build I'm using a YFM250 head, and it looks like I'd better measure the volume! I had assumed they were all the same.

I went and bought shorter bolts and copper washers so I could put the base gasket in and torque the cylinder down to get the deck height.
Did you happen to take before and after measurements? I'm curious as to how much the base gasket compressed. :dontknow:
 

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I would have assumed all heads had the same casting, but Im leaning towards they don't. Not sure if that intentional or just the way it come out. I will buy a new head if that's what it takes and I can make sense out of it.

I took a rough measurement before I squished it down and want to say it compressed about .010". I figured the uncompressed deck height wasn't worth too much to me. my crushed head gasket of .035" is based off a multi layer stainless oem Yamaha ytm250 head gasket that's too small a die. Im hoping cometic can make one for me that is .027x2.913". as of 6:05 pm im leaning towards stroking the crank to make up the deck height because you get 2X for your money that way since you're also increasing your swept volume. however you've already decked your cylinder.
 

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Discussion Starter #68
I got a stock style YFM250 gasket, and carefully dremeled it out to 74mm bore. I think it would have worked nicely (and I'll probably do it again), but I ran in to another problem.

When I put it all together with the head bolts just finger tight, everything rotated smoothly. When I torqued the head bolts down though, there was some interference at TDC. Not much... I could turn it through by hand, but still a no-go.

But wait, there's more! When I tried turning the crank back toward BDC, there was a sudden and solid stoppage. It took me a while to figure it out, but the counter balancer was hitting the bottom of the cylinder sleeve. Again, not by much... but definitely a no-go!

Both of these issues can be addressed by using a thicker base gasket (or spacer). Alternatively, I could clearance-grind the bottom of the cylinder sleeve, and use a little bit thicker head gasket. I guess I shouldn't have had that .025" machined off bottom of the cylinder after all. :rolleyes:

Either way, I'm back to looking at lower compression ratios. I think low 9s is the best I can hope for with this piston.

VGs suggestion to stroke the crank is a good one, but it's a little beyond my skill set. I'll be watching his project with interest, though. :cool:
 

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Just a stupid question but can't you just deck the head a bit for a higher compression ratio?
Reason for me asking is that I have way more experience on cars than bikes, and you can safely increase compression on a car by taking a little material off the head. If you go too far though you run into trouble with timing chain/belt tension issues.
How far can you go on a TW/XT head?
 

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Discussion Starter #70
Well yes, and that's a great point! I guess I got a little tunnel vision on this build, and since the cylinder needed work anyway, I put my focus there.

If I had not milled the bottom of the cylinder block, then I could have milled the head by the same amount, and had the same effect on the c/r without creating the interference problems I'm having now.

Now that I'm probably going to space the cylinder back up, I may mill the head and give it a try.

How much is too much? I don't know... I think the timing chain tension adjustment may be the limiting factor.

For other (smaller) cylinder bores, there is a pretty good variety of pistons available for different compression ratios. Some of them are domed pretty high, and valve clearance might become an issue when milling the head. For the 74mm bore though, we are pretty much stuck with this one piston, which is slightly dished, and meant to be used with a smaller combustion chamber (XT250, completely different head). I think the TW head could be milled quite a bit without causing any valve clearance issues with this piston.

This project began as a conversion upgrade to an existing engine to work in a TW, so I've never wanted to tear in to it very far... in fact I've been sucked in a lot deeper than I ever intended to be! ;) I just wanted to build a 250 with bolt-on parts, and it's not as easy as I anticipated. If I were to start from scratch on this engine, I'd to a complete tear-down from the get-go, and one of the things I'd do is to ream out the wrist pin bore, or find a different rod that could accept a larger wrist pin, and open up a much broader selection of pistons.

Who knows, I still may end up there before I'm done.... :unsure:
 

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Yes the timing components are the limiting factor, I know that on heads with cam pedestals you can just shim up the pedestals, but it won't work as the cam is in the head. The other and more expensive option is to weld up and then line bore the head to move the cam further away from the crank.
My best guess is 030 to 040 will be the most you can mill off before the tensioner is going to be iffy(don't hold me to it, but from what I saw I recon that to be the most).
 

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Discussion Starter #72 (Edited)
I took the cylinder back off, and did some light grinding so the counter-balancer clears even with no base gasket in place. Also, with no base gasket in place, the piston protrudes a full 1mm at TDC... so that's the amount of space I need to make up with gaskets.

I doubled up the base gasket, for a compressed thickness of 1.3mm, and the head gasket I dremeled out has a compressed thickness of about 0.9mm. In theory, this should work out pretty well. I torqued the head bolts down, and I can turn the engine all the way through by hand, with no interference.

Now I'm worried about the oil passage o-ring in the base gasket though. With a double base gasket, I'm not sure there's enough compression on the o-ring to make a tight seal. I'll have to open it up again, and maybe find a thicker o-ring... after all this work, I'd hate to fry the head due to not enough oil pressure!

While I'm at it, I'm exploring other head gasket options as well. I think I'm getting close to finishing this up... but I've been thinking that for a while now. :rolleyes:
 

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Wow...just got wind of this thread. Sure is an ambitious project...but I have to wonder if the extra 25cc are going to be worth it?
As a sidenote...you can offer up the rear wheel of the TTR to any XT225 owners...the XT rear rims are notoriously weak and many owners are looking to swap out the rear wheel since the TTR's are steel and have a better lacing pattern (supposedly). If I had found one, I'd probably have kept my XT as it was an awesome little bike!
 

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Discussion Starter #74
I'm pretty sure the extra 25cc will not be worth it.... Honestly, I think the best bang for the buck if you want more displacement and 6 speeds, is to find a good running XT or TTR engine, and swap it directly over to the TW (I'll be doing this soon, in another thread).

If you need to do cylinder or head work, then honing to 71mm and using the 10.5:1 Wiseco piston for 229cc might be a worthwhile step, and would most likely out-perform the lower compression 249cc I'm working on now.
 

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mine is a lower compression 229cc TTR with a machined TW shaft, and i can say the extra oomf is nice as is the 6spd wide-range tranny. i can hill climb in 3rd what i used to lug and have to drop into 2nd and rev the crap out of it. 1st is lower and really crawls, 6th makes short trips on the freeway a bit less hairy.

been very happy with the conversion.
 

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Discussion Starter #76 (Edited)
Well, I think it's time to wrap up this train-wreck of a thread! ;)

I ended up using one base gasket, along with the Wiseco head gasket. The Wiseco head gasket bore measures 73.8mm, while the cylinder bore measures 74.0mm. The gasket still fits over the piston, but I was afraid it might not sit perfectly centered over the bore, so I beveled the top edge of the piston just a bit.

With an uncompressed base gasket, the piston sits pretty flush with the top of the cylinder.


208074


Here it is with the head gasket set in place.
208075


Once everything is torqued down, I think the compression ratio will be around 9:1. It should make for an easy kicking, nice running 250. I'm hoping it will keep up with the higher compression 225 I built for my friend.

Oh, I also re-marked the cylinder, in case some future owner gets confused.
208076


I'll post a picture of the assembled engine when I get home, but this project is officially over!

I'll start a new thread if I ever get around to installing it. Thanks for watching! 👋
 
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