Honda CB500RS

[tevie54]

Can anybody see any reason why fitting a Wiseco 10.5 compression piston should cause problems with the skirt picking up on the barrel after only 300 miles

[johnr]

was the barrel bored to match the piston or was it just fitted to a previously used barrel? did you hone the bore before you put the piston in? was the bike getting hot, im assuming youve checked that its getting enough oil etc.

[themoudie]

Aye tevie54,

Is it a slipper piston and how old is the oil pump and it's seals? The seals harden with heat and age and as I found with Sally SRX, to my cost, big slipper pistons (Wiseco 634cc) with marginal lubrication are a recipe for 'picking up' on the barrel at the front and rear of the piston. Also, the piston ring end gaps are critical, too narrow and there is insufficient room for expansion again leading scuffing and 'picking up', if not seizure on the front and rear faces of the piston and the rings.

Would advise a rebore and correct honing by a competent machine shop as essential when fitting a new piston. In Sally's case I fitted a new liner and 'standard' piston plus hone, with no increase in compression. High compression is fine for top end 'umph' but for tractable day to day riding something a bit softer, with useable torque as the engine performance criteria is more appealing to me.

Was the bike 'lying' for a while and did you change the oil and check for debris? With the Morini, the oil looked clean, but after standing for 9 years all the clag had settled out in the sump and solidified!    Lots of flexible poking  and flushing with paraffin, until the flushing ran clear was required and then 2 oil changes, one after 20 miles and another after a further 200 miles using old style 20/50 appear to have done the trick. I would use this oil change routine for running in the engine as well and don't lug it, keep it spinning under light load and increasing revs rather than being cautious and letting it lug and thump.

Semi-synthetics have heavy duty detergents to dislodge and keep the crap in suspension and if there is a lot of it about, big lumps may block or partly fill oilways and galleries. I know splitting cases would be better, but I was being a lazy s*d!

Hope you find the problem.

My regards, Bill

[tevie54]

Yes it was strange, the barrel was bored and honed to suit the new piston, the ring gaps were checked before fitment, I had run it in carefully up to the 300 mile mark and was giving it a bit more welly when it nipped up but freed off almost immediately, it ran okay for the rest of the day but I could feel something wasn't quite right.
On inspection it had scuffed the piston skirt front and back but the barrel had escaped any damage, I rehoned the barrel and cleaned up the piston with fine emery and changed the oil pump for a better condition one, the piston to barrel clearance is correct to the instructions that came with the piston. There is too much play in the small end so the engine is coming apart soon for a crankshaft rebuild with a new con rod but I'm reluctant to refit the piston back in case I've missed something. Should I go back to a standard compression flat top piston?

[themoudie]

Aye tevi54,

My advice would be to bring forward the small end renewal, it will not be helping you maintain the clearances required for running in a new piston.

Who ever is renovating your small end, either by re-bushing the small end or replacing the con rod, should have the tools, measuring instruments and experience to advise you on the re-fitting of the now damaged piston, rings and the re-honed barrel. When the barrel was re-honed, was it done freehand or in a fixed position? You can make the bore of the barrel not square to the base of the barrel and or face of the crankcase and movement of the conrod and this increases the side thrust on the piston.

Having used fine emery on the piston make sure you wash the piston in hot water with degreaser, rinse thoroughly and the dry, so that no abrasive particles are on the piston afore re-fitting the rings, which also need to be spotless. I would also suggest a new pair of circlips for retaining the gudgeon pin, when rebuilding.

If the piston is of a 'slipper' design then the area of skirt to support the piston may have been reduced when compared with a full skirt piston. Hence the tendency for these pistons to tip in the bore and potentialy have increased wear on the front and rear skirts. Less friction, less weight, higher piston speed, more power, increased wear.  Poke is never free!

Providing the 10.5:1 piston is servicable, go ahead and use it and enjoy the difference that it may make to the engine characteristics. It just might not last as long as that 'standard' piston. 

I hope that you soon have the motor running again.

My regards, Bill

[guest564]

Did you have the barrel bored to the clearances specified by Wiseco? from memory they are a little larger than the Honda spec.
The higher compression will give more power throughout the rev range, high compression engines are often considered to be 'revier' but that's because they are usually fitted with higher performance camshafts.

[tevie54]

That is what I am wondering, the machine shop may have bored and honed to a standard piston spec, I have since honed to the correct Wiseco recommended clearance of two thou so we'll see what happens when it all goes back together. I really want to use the high comp piston as I dont think 10.5 to one is excessive compression and with the mild porting job and the XR cam it should be just quick enough but reliable as well.     

[themoudie]

Aye tevi54,

Finding another machine shop as well? 

I hope you sort it out.

My regards, Bill

[steveD]

That is what I am wondering, the machine shop may have bored and honed to a standard piston spec, I have since honed to the correct Wiseco recommended clearance of two thou so we'll see what happens when it all goes back together. I really want to use the high comp piston as I dont think 10.5 to one is excessive compression and with the mild porting job and the XR cam it should be just quick enough but reliable as well.   

TWO THOU!!!!!!!!!!!!!!!!!!

What does that refer to. I only say this because when I was a Toolmaker and used to do my own rebores, the rule of thumb was 3 thou per inch of bore (overall).
I don't think that 2 thou would be sufficient for the piston to expand before the liner and barrel expands.

Just my ten pence worth.

Steve

[Dave#22]

Hi Steve, .....0.002" may be on the tight side, but the DR750/800 with a 105mm (std) piston..... recommended piston to cylinder clearance is 0.0022" to 0.003"...cast or forged!
I always thought the clearance was too little, but I've done thousands of miles on them and never had an issue.
Dave.

[johnr]

Hi Steve, .....0.002" may be on the tight side, but the DR750/800 with a 105mm (std) piston..... recommended piston to cylinder clearance is 0.0022" to 0.003"...cast or forged!
I always thought the clearance was too little, but I've done thousands of miles on them and never had an issue.
Dave.

aye, and the dr big engine was designed to run flat out at high revs for hundreds of miles at a go across the deserts of north africa and they didnt ail much from it.

[steveD]

So that's .006" (six thou) overall which is half of what I would have done! But then I was machining 40 years ago and we all know that Japanese machinery is tighter on tolerances.

Still pretty impressive.

Steve

[tevie54]

You've got me thinking now, it says on the box clearance should be 0.0020 but does that mean overall I.E. the piston pushed over to one side or does it mean both sides so overall 4 thou, bear in mind i'm only measuring the skirt front and back as there isnt any skirt at the sides.

[Propellor]

.

[Dave#22]

Hi tevie 54, it will be 0.002" with one side of the skirt against the cylinder and a feeler gauge between the other skirt and the cylinder.

  Steve, It's not 0.006", but 0.0022" to 0.003" overall.

DR750/800 spec.

Bore size.....4.1339"to 4.1344" (105mm)
Cast piston dia DR750........4.1319" to 4.1327 = 0.0015" to 0.0022" clearance.
Forged piston dia DR800.....4.1311" to 4.1319 = 0.0022" to 0.003"  clearance.
Wear limit....0.0047"

From Suzuki service manual.
Dave.