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Tappets, tight on one cylinder, loose on another?

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EazyDuz
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Joined: 11 Apr 2011
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PostPosted: 21:22 - 07 Jul 2020    Post subject: Tappets, tight on one cylinder, loose on another? Reply with quote
Finished doing the valve adjustment on my C800 V twin today.

Each cylinder has two exhaust valves and two intake valves.
Specs are 0.03-0.05 inch intake. 0.07-0.09 inch exhaust.

These were my findings, using the 'go no go' method:

Front cyl:
Exhaust - both too tight
Intake - both too tight

Rear cyl:
Exhaust - both too loose
Intake - both in spec but on the tighter side.

Just curious why the exhaust on one cylinder would have got a wider gap over time whereas the front cylinder the gap closed up over time?

Adjusted them all and fired the bike up and its running great
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Easy-X
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Joined: 08 Mar 2019
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PostPosted: 21:48 - 07 Jul 2020    Post subject: Reply with quote
Unless you're talking CX500 or Moto Guzzi I'd be very surprised if your valve clearances from front to back weren't different - the front's always gonna dump heat better than the rear Smile

As to the how of valve clearances changing over time you'll need a materials engineer. There'll be one along shortly...
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steve the grease
Crazy Courier



Joined: 26 Jan 2018
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PostPosted: 14:17 - 08 Jul 2020    Post subject: Reply with quote
Just to be a pedantic twat , I think you will have missed out a decimal point. 30-50 thou on the inlet 70-90 thou on the exhaust is definitely not right. 0.003-5" in/ 0.007-9" exh is rather more likely. It's that conversion thing from metric. Millimeters are hundredths - inches are thousandths.
so 1mm =40 thou (39.37)= 0.040"
0.1mm= 0.004"
and so on.

I'll get me coat/ micrometer/vernier caliper.
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jeffyjeff
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Joined: 02 May 2020
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PostPosted: 01:52 - 09 Jul 2020    Post subject: Reply with quote
Easy-X wrote:
As to the how of valve clearances changing over time you'll need a materials engineer. There'll be one along shortly...

The mechanical engineer is out to lunch, but he saw your post and asked me to help out. ha ha. I'll try a brief explanation.

The exhaust valves are the hottest moving parts of your engine, followed by the intakes. Over the millions of cycles between service intervals, the valve face and seat wear into each other. This effectively causes the valve to recede into the cylinder head, reducing clearance between the rocker arm and valve stem. It is normal for valve clearance to decrease over time.
How can the clearance increase? The C800 rockers have a wide, flat shoe (opposite the adjuster) that the cam acts upon. Wear on the rocker shoe, or wear on the cam lobe can cause the valve clearance to increase. Wear on your screw adjusters can also increase the clearance.

In a perfect world, wear on the cam and follower is exactly offset by wear at the valve face and seat, and the engine can go many thousands of miles without adjustment.
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EazyDuz
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Joined: 11 Apr 2011
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PostPosted: 08:14 - 09 Jul 2020    Post subject: Reply with quote
jeffyjeff wrote:
Easy-X wrote:
As to the how of valve clearances changing over time you'll need a materials engineer. There'll be one along shortly...

The mechanical engineer is out to lunch, but he saw your post and asked me to help out. ha ha. I'll try a brief explanation.

The exhaust valves are the hottest moving parts of your engine, followed by the intakes. Over the millions of cycles between service intervals, the valve face and seat wear into each other. This effectively causes the valve to recede into the cylinder head, reducing clearance between the rocker arm and valve stem. It is normal for valve clearance to decrease over time.
How can the clearance increase? The C800 rockers have a wide, flat shoe (opposite the adjuster) that the cam acts upon. Wear on the rocker shoe, or wear on the cam lobe can cause the valve clearance to increase. Wear on your screw adjusters can also increase the clearance.

In a perfect world, wear on the cam and follower is exactly offset by wear at the valve face and seat, and the engine can go many thousands of miles without adjustment.


Interesting, thanks for the detailed reply.
Service interval is every 7500 miles on the tappets but I don't see an issue with doing it every 10,000. Yes some valves were tight but had no hot starting problems and the bike was running just fine before hand. It was just nagging me not knowing when the last valve job was done as it's not in any of the previous owners service records.
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Easy-X
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Joined: 08 Mar 2019
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PostPosted: 12:07 - 09 Jul 2020    Post subject: Reply with quote
Worth remembering that valve gaps are there to allow for heat expansion. If a gap is a bit tight you probably wouldn't notice a valve holding open but it would put slightly more wear on a cam or pushrod.
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jeffyjeff
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Joined: 02 May 2020
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PostPosted: 13:54 - 09 Jul 2020    Post subject: Reply with quote
Easy-X wrote:
Worth remembering that valve gaps are there to allow for heat expansion.

You are right. Valve face to seat contact is what cools the valve. If the gap is too tight, the engine may actually run smoother, but insufficient clearance will reduce face/seat contact time and the valve will overheat. In severe cases, the hot exhaust gasses will burn the valve and the cylinder would lose compression.
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Pete.
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Joined: 22 Aug 2006
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PostPosted: 19:20 - 09 Jul 2020    Post subject: Reply with quote
Easy-X wrote:
Worth remembering that valve gaps are there to allow for heat expansion. If a gap is a bit tight you probably wouldn't notice a valve holding open but it would put slightly more wear on a cam or pushrod.


The gap is there for several things. Heat expansion, cooling of the valve head via the seat, maintaining an oil film on the cam lobe.
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