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Laser Engines - Technical questions

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Paul james 818/11/2019 23:13:07
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158 forum posts
47 photos

laser 90 dirtyAn old 90 refurb, bearings really bad, all very dirty but soon sorted out, marks on the piston crown where the valves had touched but thankfully neither was bent and it all went back together.

laser 90 crankcase

laser 90 head

laser 90 piston

laser 90 dirty

Bearings!!

laser 90 bearings

laser 90 cleaned

Both of these are 90's, the bigger one to the left is an earlier, longer stroke model.....

two 90s.jpg

Edited By Paul james 8 on 18/11/2019 23:15:18

Ron Gray19/11/2019 05:31:52
1550 forum posts
367 photos

Hey Paul, there’s some dirt on the exhaust on the one on the right. wink 2

David Davis19/11/2019 06:14:45
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3465 forum posts
622 photos
Posted by Chris Walby on 18/11/2019 21:23:13:

The rogues gallery should be a list of people who have bought engines and are not using them.

On an annual basis owners should declare the number of hours the engine/s have run and if not sufficient then returned to Jon for resale.

I am guilty of an 80 in a box for a couple of years, but it will be in a ARTF for next year honest....deviland if I ever get time I'll build the model it was intended for.

I plead guilty to owning eight m'lud: a 62, three 70s, an 80, a 90, a 150v and a 155, none of which are currently installed in a model in flying consition.

In mitigation, the 62 or one of the 70s will be installed in the Big Guff I am building; another of the 70s will go into an ARTF Acrowot; the 80 will go into a DB Sport and Scale Auster which only needs the fin, the rudder and the struts to be built and covered and the 150v is already installed in a WOT 4 XL which needs a recover! blush

Jon - Laser Engines19/11/2019 08:26:07
4894 forum posts
186 photos

Paul your short 90 is actually an 80, or perhaps a very late 75 like your other example.

The 80 was just a bored out 75 and the 100 was a bored 90. A simple check is to see if the piston from the 90 fits, if so its a 75, if not its an 80

I was trying to see if there was anything stamped on the mounting lugs that would give a date of mfr and help pin down the size but the photos dont show it.

Paul james 819/11/2019 09:23:22
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158 forum posts
47 photos
Posted by Jon - Laser Engines on 19/11/2019 08:26:07:

Paul your short 90 is actually an 80, or perhaps a very late 75 like your other example.

The 80 was just a bored out 75 and the 100 was a bored 90. A simple check is to see if the piston from the 90 fits, if so its a 75, if not its an 80

I was trying to see if there was anything stamped on the mounting lugs that would give a date of mfr and help pin down the size but the photos dont show it.

Interesting comments Jon, I put it down to be a 90 as I carefully measured both bore and stroke while it was apart for refurbishment and it came up at 90?

Could it be an experimental one perhaps?

Edited By Paul james 8 on 19/11/2019 09:23:46

Chris Walby19/11/2019 10:01:05
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1025 forum posts
238 photos

If in doubt measure the bore and stroke and calculate the volume, job done!

Paul james 819/11/2019 10:10:44
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158 forum posts
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Posted by Chris Walby on 19/11/2019 10:01:05:

If in doubt measure the bore and stroke and calculate the volume, job done!

Erm, that is what I did. wink

Jon - Laser Engines19/11/2019 10:27:41
4894 forum posts
186 photos

If memory serves the calculation for capacity is pi/4 x bore^2 x stroke.

Our 90, 100 and 150 all share the same stroke and just had a variety of bores. The same was true of the 75, 80, and 120.

The smaller engine cant be a 90 as the crankshaft will not physically fit in that crankcase although the 85 and 90 share a bore as do the 80 and 100.

So if we assume you got to a capacity of .9 by multiplying bore and stroke in inches to get

Capacity = Bore x Stroke

Stroke is constant so we get

.9 = B x .87

Work all that out and bore is 1.03 inches.

If we ignore all the rounding errors that is pretty close to the 1.04 bore size of a 75 and 90. An 80 has a bore of 1.09.

If we go back to the start and calculate capacity..

3.142/4 x 1.04^2 x 0.87 = 0.739 cu/in

call that 74 in reality but 75 sounds better for marketing

If so you have a pair of very late 75's as we didnt do many with the clips holding the carb and exhaust in place. Either of those, or the real 90 would pull the bristol without any trouble.

 

Now, time for a lie down after all that maths  

Edited By Jon - Laser Engines on 19/11/2019 10:29:01

Paul james 819/11/2019 10:44:32
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158 forum posts
47 photos

I've always used pi r squared x height for volume of a cylinder, I am intrigued now so will have to take the head off and measure again. Lol

Chris Walby19/11/2019 10:48:54
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1025 forum posts
238 photos

Well at least its not a twin overhead cam, chain drive with vernier cam wheels, V twin, oh the fun! LOL

Edited By Chris Walby on 19/11/2019 10:49:15

Jon - Laser Engines19/11/2019 11:03:04
4894 forum posts
186 photos
Posted by Paul james 8 on 19/11/2019 10:44:32:

I've always used pi r squared x height for volume of a cylinder, I am intrigued now so will have to take the head off and measure again. Lol

That works for cylinder volume but not engine capacity

plugging .87 and 1.04 into this gives me the same .74 result so im not totally mad **LINK**

Manish Chandrayan19/11/2019 12:35:15
606 forum posts
70 photos
Posted by Paul james 8 on 19/11/2019 10:44:32:

I've always used pi r squared x height for volume of a cylinder, I am intrigued now so will have to take the head off and measure again. Lol

That perfectly works for calculating the swept volume as long as we know the bore and the stroke.

For example if the laser 75 had a bore of 26.42 mm (radius of 13.21 mm) and stroke of 22.22 mm

Applying the formula give us a volume of 12.18147 cc

Or in other words a radius (of bore) of 0.52 inches and stroke of 0.875 inches will give you a swept volume of 0.74 cubic inches 

Edited By Manish Chandrayan on 19/11/2019 12:37:44

Edited By Manish Chandrayan on 19/11/2019 12:38:41

Chris Walby19/11/2019 13:30:17
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1025 forum posts
238 photos

If you don't want to measure bore and stroke you can always do it by swept volume, but would seem a lot more effort in my book.

  1. Slacken valve adjusters or remove rockers
  2. Remove plug and rotate crank to TDC
  3. Fill cylinder head with oil
  4. Rotate crank to BDC
  5. Measure quantity of oil to fill back to operation 3
  6. Done!

Reassemble and find missing oil that has leaked passed the piston ring!

Jon - Laser Engines19/11/2019 13:51:33
4894 forum posts
186 photos

Or you could just read the number off the crankcase...oh, wait..

This is why i stamp sizes on current engines

will -019/11/2019 14:15:00
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582 forum posts
19 photos

Jon you're taking away the mystery of it all

However, good to hear something happening to "current engines" if you know what I mean.

Paul james 819/11/2019 14:46:58
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158 forum posts
47 photos
Posted by Jon - Laser Engines on 19/11/2019 11:03:04:
Posted by Paul james 8 on 19/11/2019 10:44:32:

I've always used pi r squared x height for volume of a cylinder, I am intrigued now so will have to take the head off and measure again. Lol

That works for cylinder volume but not engine capacity

plugging .87 and 1.04 into this gives me the same .74 result so im not totally mad **LINK**

Seriously, there was me thinking that engine capacity was the swept volume???

Ergo piston radius squared x 3.1417 (near enough to pi for our purpose) x stroke.

Assuming for the sake of argument a bore of 20 mm and a 20mm stroke the c.s.a of the piston would be 100 x 3.1417 so 314.7 square mm. Multiply that by 20 would give 6295 cu mm so divide by 1000 to get cc equals 6.295.

That converted to cu inch is 0.384

The only way I've ever used to find engine capacity.

Edited By Paul james 8 on 19/11/2019 14:49:35

Paul james 819/11/2019 14:47:01
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158 forum posts
47 photos

Sorry duplicate post

Edited By Paul james 8 on 19/11/2019 14:48:08

Manish Chandrayan19/11/2019 14:50:21
606 forum posts
70 photos
Posted by Jon - Laser Engines on 19/11/2019 13:51:33:

Or you could just read the number off the crankcase...oh, wait..

This is why i stamp sizes on current engines

By far the best method of all. I would go with thisteeth 2

Paul james 819/11/2019 14:55:53
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158 forum posts
47 photos
Posted by Manish Chandrayan on 19/11/2019 12:35:15:
Posted by Paul james 8 on 19/11/2019 10:44:32:

I've always used pi r squared x height for volume of a cylinder, I am intrigued now so will have to take the head off and measure again. Lol

That perfectly works for calculating the swept volume as long as we know the bore and the stroke.

For example if the laser 75 had a bore of 26.42 mm (radius of 13.21 mm) and stroke of 22.22 mm

Applying the formula give us a volume of 12.18147 cc

Or in other words a radius (of bore) of 0.52 inches and stroke of 0.875 inches will give you a swept volume of 0.74 cubic inches

Edited By Manish Chandrayan on 19/11/2019 12:37:44

Edited By Manish Chandrayan on 19/11/2019 12:38:41

It does work perfectly, the swept volume of an internal combustion engine is its swept volume. 👍😊

If Jon's figures shown above are put into the equation I use the same results are achieved.

Martin Harris19/11/2019 15:27:50
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8951 forum posts
221 photos

As far as I'm aware, engine capacity has always been quoted as the cross sectional area of the bore times the stroke. Chris's method is normally used on multi cylinder engines being "blueprinted", for balancing the cylinder head "squish" volume, which is due to the clearance between the piston and head and will result in slightly more volume than the theoretical engine capacity.

Edited By Martin Harris on 19/11/2019 15:31:16

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