High Comp. Pistons

[nukes]

stripping the top end of the rex down next weekend and chasing some advice. A mate of mine suggested going for high compression pistons to give some extra oomph. The problem is there is 137000 ks on the rest of the motor. Is this a good idea or is there the chance it may put unwanted strain on the bottom end and cause a whole lot of unhappiness. Is there much to be gained by doing it and is it worth the risk? Any thoughts and advice greatly appreciated!!!

[Max]

I for one am not to keen on the idea. Many of the tweaked TRXs that raced over here late nineties grenaded with broken conrods / cranks. I've gotten to the point where I figure a few extra ponies out of the bike isn't going to make it 'that' much quicker when you consider the possible risks.

[honkdawillydahonk]

I've got 878cc High compression pistons fitted (so has PHuk) I reckon it's a worthwhile mod if you're freshening up the top end - plus it's less risky than just skimming the head or barrels as you don't risk valve to piston contact problems....

[HansJ]

I'm en route to fit 878 pistons myself, but I am also a bit wary of the high compression causing excessive wear... I know the compression will give more performance, but I am really after more torque/midrange, and last winter I noticed the cylinder wear was on the limit. (New rings cured the oilthirst, but it's a temporary solution.)

-Would it be a good idea to decrease the compression by fitting a somewaht thicker headgasket, or will this spoil the squish too much? Maybe take some material off the pistons, or am I simply worrying too much (say: "Yes" )?
-Carillo rods, worth the investment, or just balancing and shoot-peening (is it called that?), or just use the original rods as they are?
-Balancing/reprofiling the crank? I guess this means splitting the engine...
-Is it possible to remove the rods without splitting the engine?
-What about the cam "bearings" (I know it's not a bearing, but don't know the expression in English...), i.e. the area where the camshafts rotate in the head. I could feel some wear in these. Now I've read in the German forum that they just polish the area with either some paste like the stuff used for valves or just some . Any other suggestions for refreshing the cam "bearings". Anyone considered putting in bearings similar to the halves used for crankshaft bearings?

[phuk72]

High compression pistons is the wrong way to look at it - higher would be more accurate. With pistons alone (and no skimming of the barrels / head) compression is only raised to about 10.5/1 - not exactly what I would call high!

Power is increased noticeably in the midrange but you get gains throughout.

I considered different rods but frankly it is cheaper to buy a new engine if a rod fails! And it is a big IF. Ignoring racers in Japan, has anyone ever encountered rod failure on a TRX - especially on a road bike. I would and am quite happy using standard rods.

As for the crank - again the only failures I know of are lack of oil related and so I wouldn't bother getting them balanced - and if you do you may as well get them knife edged and polished as well - and if you are thinking about that yoou have got too much money!

[HansJ]

...I wouldn't bother getting them balanced - and if you do you may as well get them knife edged and polished as well - and if you are thinking about that yoou have got too much money!

Nah, I wish it would be true, but the money I save in insurance costs compared to a CXZRRRR covers the costs easily in less then one year. If it's the wisest way to spend money on a 10 year bike, which in any case will be overtaken by any 600 on the straights (and ín the curves due to my shortcomings), can be discussed, but it's my hobby...

I am not considerìng balancing because of crank failure risks, it will reduce power losses and give a smoother ride as well. We'll see what the costs will be, right now I am just in the planning/dreaming phase...

Thanks for the thoughts regarding the rods, that's what I've been thinking as well!

[nukes]

thanks all for the thoughts. Ive decided to stick with standard pistons for the minute and maybe update the rex for a newer model that hasnt copped quite as much abuse.

[wicky]

I'm en route to fit 878 pistons myself, but I am also a bit wary of the high compression causing excessive wear... I know the compression will give more performance, but I am really after more torque/midrange, and last winter I noticed the cylinder wear was on the limit. (New rings cured the oilthirst, but it's a temporary solution.)

-Would it be a good idea to decrease the compression by fitting a somewaht thicker headgasket, or will this spoil the squish too much? Maybe take some material off the pistons, or am I simply worrying too much (say: "Yes" )?
-Carillo rods, worth the investment, or just balancing and shoot-peening (is it called that?), or just use the original rods as they are?
-Balancing/reprofiling the crank? I guess this means splitting the engine...
-Is it possible to remove the rods without splitting the engine?-What about the cam "bearings" (I know it's not a bearing, but don't know the expression in English...), i.e. the area where the camshafts rotate in the head. I could feel some wear in these. Now I've read in the German forum that they just polish the area with either some paste like the stuff used for valves or just some . Any other suggestions for refreshing the cam "bearings". Anyone considered putting in bearings similar to the halves used for crankshaft bearings?

Hi, i was wondering about this too, i have been told it is possible by removing the sump cover, oil strainer, etc and using a long socket extension bar to unbolt the big end of the rods. I have got me hands on a set of TDM 900 pistons, rings & conrods (low miles, 3k from new !) Yamaha only want £284 including VAT ferra brand new set of TDM 900 barrells so this is the way im goin to go eventually..............just gotta find 284 smackeroonies now...... best of luck wIcK

[phuk72]

I don't think you can remove the con rods without splitting the cases (but then again I have never actually tried).

Have to say I am interested in whether you can but I would personally drop the engine out (not especially difficult but borrow a mate as the TRX engine is the heaviest lump I have ever come across) and split the cases.

Con rods are pretty important and I would want to take any risks with fittng so I would split the motor down. Just me ...

How much are the pistons and rings btw?

[wicky]

I don't think you can remove the con rods without splitting the cases (but then again I have never actually tried).

Have to say I am interested in whether you can but I would personally drop the engine out (not especially difficult but borrow a mate as the TRX engine is the heaviest lump I have ever come across) and split the cases.

Con rods are pretty important and I would want to take any risks with fittng so I would split the motor down. Just me ...

How much are the pistons and rings btw?

PHUK i didn't price up new pistons and rings, but someone mentioned a while back that pistons were around £125 each !!!

[phuk72]

That'd be about right - £500 or so.

You know that you can get a 878cc big bore kit and have the barrels bored for less than that.

Saves all the arsing about having the internals modded to take the TDM900 rods as well.

Power wise, the 878 kit probbaly just takes it but this is mainly due to the higher comp.

[jpe]

I don't think you can remove the con rods without splitting the cases (but then again I have never actually tried).

Actually you can (or so I read). But it takes a wee bit o' fiddling doing it apparently. Don't wanna contemplate putting 'em back again with the case in one piece...

[nukes]

hey phuck, price for pistons over here are $110 each and rings $75 each.

[wicky]

That'd be about right - £500 or so.

You know that you can get a 878cc big bore kit and have the barrels bored for less than that.

Saves all the arsing about having the internals modded to take the TDM900 rods as well.

Power wise, the 878 kit probbaly just takes it but this is mainly due to the higher comp.

all good/valid points PHUK but im out to cure oil consumption and this way (the 900 conversion) is proven to be long term effective, unlike a standard rebuild or oversize pistons which is good for prolly 15-20,000 miles before the oil consumption starts all over again. The interesting (and hopefully if it works, less hassle) ting is a friend of mine is looking into the possibility of using stock TDM/TRX 850 barrells and getting the liners pressed out, overboring and fitting alloy liners, and then having the alloy liners ceramic/nikasil coated like the TDM 900's are supposed to be, this allows ferra more even/uniform thermal expansion between the pistons/bores allowing a tighter piston/bore clearance, which helps retain more oil. A suitable size piston like 878 fer example could then be used if required. i can't remember the piston/bore clearance fer 850's but i do remember the TDM 900 piston/bore clearance was significantly less, hence they do not burn/use any oil atall atall even on engines with 40,000 + miles. if me mates idea were successful it'd be easier than doing the 900 conversion which requires machining conrod little ends, skimming barrells, cutting out gaskets, and timing the thing up mainly due to the 900's concave/dished piston crowns which rob compression. watch this space i suppose



update; it looks like the idea mentioned above to replace the liners & plate them is proving problematic in application and most of all price so i have been advised to save me pennies fer 900 barrells instead !

[PhatVR6]

I'm looking into boring/piston options at the moment.

Who makes the 878cc pistons and where do I buy them? what do the barrles need boring to or are the specs all in with the piston kit? is this as far as we can go bore wise?