Yamaha TRX850 Enthusiasts Forum

anyone any idea how much needs skimming off the top of the barrel to get the squish correct with STANDARD YAMAHA pistons. i am aiming for 0.030'', i have set this gap on dozens of bikes.

1.2mm (0.048'')is NOT a squish clearance it is a gap.....unless you are revving real hard maybe 10K up

i set my brothers FZR1000 up with a 0.030'' squish as per loctite yamaha specs and picked up an incredible 14hp rear back to back same dyno.

Erm doesn`t an FZR1000 rev past 10K?

TTs told me a short while ago that as a guide when I was measuring/setting squish on my JE878 motor that anything less than 48 thou may result in piston/head contact. Measured up to be spot on with no skimming required using standard (tho modified) Yamaha head and base gaskets..

91mm pistons needing a larger clearance than a high revving multi cylinder machine makes sense to me but I may well be wrong.

Saying that I did not measure it with the stock pistons so it may have differed from stock?

Mine flies set at 1.2mm and no evidence of piston contact with sustained track day abuse (only revving to 8k max) so I'm happy.

Thought I had better acknowledge that you specified STANDARD pistons.

Compared with the standard pistons the JE878 items have a mild dome to up the compression ratio and they do have decent sized valve cutouts.



Piston crown height may be increased over stock but piston mass was only about 30g's different.

Probably no help to you what so ever!!

Let me know if 30 thou works and I'll do the same mod to mine. Might save me from forking out for FCR's!

Hmmm, if Rich Albans at TTS says 48 thou I would personally trust the man`s judgement. He`s sold and installed plenty of JE 878cc kits in his time, he does know what he`s talking about.
Check just how little 30 thou is with a feeler blade, that`s just 0.762 mm, a shade over 3/4 of a millimetre, 48 thou being 1.2192 mm, a much more `sensible` and safer clearance.

thanks for your input mincehead, you are dead right an FZR1000 does rev past 10,000rpm. but i wasnt refering to an FZR when i said " if you are revving to 10K. please read again. They were seperate statements.

i will go with 0.030 as a squish. i just needed the deck height which nobody knows.

i will remove cyls and measure. just would have saved some time. could have got the cyls decked beforehand.


1.2mm is a GAP

I am sure one of our fellow members must have/is a Mechanical Engineer. Cosworth/Ricardo etc all set there squish very low.

What fuel are you planning on running, super unleaded 98 RON? At a 30 thou squish you may find power sapping detonation creeps in to the equation, and you may also find the stock Yamaha rods crying no more with very high compression ratio`s too.
Members here have had the stock rods let go on tuned TRX motors.
You`ll also be aware that piston to bore clearance is another factor? ALL pistons `rock` in a bore as they pass TDC, this will also affect the squish clearance.

I agree Mince. It was explained to me that beyond the simple static measurement of piston/head clearance, it is indeed the extent of any rock of large pistons that has to be considered.

Mince

if i skim the cylinder 0.020-0.030 which is about the maximum it will need to achieve 0.030 squish the compression will go up about 1/2 a point, if the Yamaha 10.4:1 is to be believed which is normally quite exagerated then at most i would have 11:1 which is not high at all. the smaller the squish clearance the MORE is suppresses detonation, this is the whole point of a squish band. I am not doing this blind, i have a lifetime of building engines for fun and professionally, and a degree in Engineering. What i was trying to find out initially is the deck so i can skim cylinder before strippping the motor

The piston rocking in the bore would/will be worse the greater the piston/bore clearance, BUT as the stock piston have a COLD clearance of 11/2 to 21/2 thou, which when warm will close up this is not a real problem. with a scientific calaculator and simple Trigonometry the "rock" at the front and back of the piston can be calculated, but as the radius is 45mm and the cold clearance 21/2 thou, it will not be much.

I hear what you say regarding rods but again am perplexed, the main cause of rod failure is increased inertia forces caused by either heavier pistons or higher revs both of which increase the stress, the stresses actually increase as a square of the engine speed, so for instance an engine that revs to 8K will have a stress of 64 whilst an engine revving to 9K will have a stress of 81. a 26% increase in stress for a 12.5% increase in RPM. The higher revs cause the mass of the accelearating piston to have such inertia that the rod breaks, think for a minute when a rod breaks which valve is damaged??
It is always the EXHAUST because there is no cushion of gas above the piston to slow the accelearating piston. Just increasing the compression will increase a compressive loading which metals prefer to extension forces.

I am aware of the history of Richard Albans from TTS and have dealt with him in the past. It maybe that the squish he is suggesting of 1.2mm( 0.048) is something he has form J&E for thier 878 kit, but for cast Yamaha pistons i believe the figure is high. I will set mine at 0.030 as discussed, and go fromthere, I will do a back to back dyno run for the benefits of the foruma nd we can see the change for the ££££

thanks for your input

Hi

I have set my TDM 850 to 0.8mm with standard pistons by skimming (if I remember correctly) 0.8mm from the top of the cylinders and she's done nearly 4000miles with no problems whatsoever , and she gets a right good pasting on Sunday blasts trying to keep up with my mates on their KTM's.
I had the head off around 800miles ago to replace a gasket and their was no evidence of any piston to head/valve contact whatsoever, but dont forget when you skim the cylinder you are effectively making the cam chain longer so you will have to slot the cam sprockets and advance both cams and also extend the tensioner depending on the state of your chain , but its well worth the effort ,when she reaches 5000rpm it just dives for the redline almost 2 stroke like! I have done the TPS mod and I'm always hitting 9000rpm by accident.
HTH.

BigKev

thanks for your input, i knew we would reach some common sense at some point.

0.8mm =0.032 and you are experiencing NO piston contact at 9000rpm.

i will do as i felt right, follow your lead thanks a lot

Your welcome

Hope I dont get flamed for riding a TDM and lurking on a TRX forum

BigKev

if its any help i also have a TDM but the tdm owners on the forum dont tune thier bikes.....

Let us know how it all goes?

howardsmed wrote:Mince

if i skim the cylinder 0.020-0.030 which is about the maximum it will need to achieve 0.030 squish the compression will go up about 1/2 a point, if the Yamaha 10.4:1 is to be believed which is normally quite exagerated then at most i would have 11:1 which is not high at all. the smaller the squish clearance the MORE is suppresses detonation, this is the whole point of a squish band. I am not doing this blind, i have a lifetime of building engines for fun and professionally, and a degree in Engineering. What i was trying to find out initially is the deck so i can skim cylinder before strippping the motor

The piston rocking in the bore would/will be worse the greater the piston/bore clearance, BUT as the stock piston have a COLD clearance of 11/2 to 21/2 thou, which when warm will close up this is not a real problem. with a scientific calaculator and simple Trigonometry the "rock" at the front and back of the piston can be calculated, but as the radius is 45mm and the cold clearance 21/2 thou, it will not be much.

I hear what you say regarding rods but again am perplexed, the main cause of rod failure is increased inertia forces caused by either heavier pistons or higher revs both of which increase the stress, the stresses actually increase as a square of the engine speed, so for instance an engine that revs to 8K will have a stress of 64 whilst an engine revving to 9K will have a stress of 81. a 26% increase in stress for a 12.5% increase in RPM. The higher revs cause the mass of the accelearating piston to have such inertia that the rod breaks, think for a minute when a rod breaks which valve is damaged??
It is always the EXHAUST because there is no cushion of gas above the piston to slow the accelearating piston. Just increasing the compression will increase a compressive loading which metals prefer to extension forces.

I am aware of the history of Richard Albans from TTS and have dealt with him in the past. It maybe that the squish he is suggesting of 1.2mm( 0.048) is something he has form J&E for thier 878 kit, but for cast Yamaha pistons i believe the figure is high. I will set mine at 0.030 as discussed, and go fromthere, I will do a back to back dyno run for the benefits of the foruma nd we can see the change for the ££££

thanks for your input

Any chance of an update on your progress so far



Mark.