Yamaha TRX850 Enthusiasts Forum

Std cam valve clearance is give as
Inlet. 0.15-0.2mm

Exhaust. 0.25-0.3mm
I would imagine the higher exhaust clearance would be due to the expansion from hot exhaust gases.

But the Kent cams specs give a clearance of 0.15mm for both exhaust and inlet???
I'm curious why this would be???

Phone call to the UK last nite and the tech staff from Kent Cams at in Birmingham for a car show till Monday... Bummer

Brian,

I wouldn't waste money phoning the U.K.

I think you'll find that because the engine has cams that run directly on valve centre line, cam lift minus nominal valve clearance = valve lift. Obviously this does not hold true for S.O.H.C engines with rockers (like my SZR660)

In any case, for our engine, setting cam timing using lobe centre angle will work regardless of valve clearance. (The point of maximum lift remains unaltered whether the clearance is zero or massive)
When I built my engine I set all my valve clearances as per Yamaha specs with the head on the bench.
Cams were timed up after slotting cam sprockets. I removed the cam chain tensioner before making any adjustments. I also used brightly coloured cable ties to temporarily retain the chain to the sprocket to stop the valve spring pressure trying to turn the cam sprocket, cutting these ties off when they had served their purpose.

Mark.

Thanks Mark , just curious why Kent cams would spec a 0.15 mm exhaust valve clearance value which differs from Yamaha??? Obviously Yamaha spec 0.25-0.3mm for a reason..
Maybe its oversight from Kent and needs to be corrected???
I noticed that when the head was set up and handed to me John used the Kent specs as they are all within 0.15-0.18mm on inlet and exhaust.

It may be because of cam profile and how it picks up the bucket coming onto the ramp. I would go with the Kent clearance until I was sure it was a "typo".

I checked out Kent's website for other Yamaha direct acting valvegear. The clearances are pretty varied, mainly with a larger exhaust clearance but some (R1) models have equal inlet and exhaust clearances. I reckon Kent have got it right - but I agree, normally the exhaust is larger because it gets hotter but older engines like the original Mini had same clearance for both.

Thanks guys, I'll be patient and make contact next week to find out the reasoning.
It may be as Rich said, due to the ramp angle.
If i get it from the horses mouth I'm sure they will explain why.

I noticed Mark had mentioned in conversation with Kent cams that the extra lift is 0.43mm on the inlet and only 0.28mm on the exhaust.

But specs from Kent give 8.43mm lift on both inlet and exhaust. (Std TRX 8mm)
So if you add the extra 0.15mm clearance in question on the exhaust to Marks figure , this gives you the 8.43mm lift.
I've sent an e mail so hopefully I'll have answers to these questions by next week.
I reacon your on the money with the ramp angle Rich
Maybe the exhaust doesn't have the same height lift but it may have a different profile, shelving at the top of the lobe and not peaking like the inlet.

After writing that last post , I checked my e mail and there is the answer from Tony Woodward.
Yes your correct Rich,
All to do with the ramp angle so that's why the clearance will be as per Kent specs is 0.15mm.
Both inlet and exhaust profiles are the same.
Here is the response below.

Hi,

The YAM9 is the same profile inlet and exhaust so they will have 8.43mm on both.
The valve clearance is 0.15mm for the design of the ramp on the profile.
The 240 degree duration we quote is at a checking height of 1mm with zero clearance.
The duration quoted by Yamaha will at a different height.
The easiest way to time them is on the 105 / 109 lobe centre angles.

Regards

Tony Woodward.