Laser 61 after 30+ years

[Frank Skilbeck]

Last year I replaced the bearings on my Dads old Laser 75 that he'd bought in the late 1980s, done many years service and was taken out of his old Majestic Major last year top replace the noisy bearing, an old 62 went back in the Major and continues to run well, so I recently put it in an old low wing model, again one my Dad had built years ago and I'd test flown for him. The 75 seems down and compression and while it ran well was only getting around 8,000 rpm on a MK 13 x 6, I recently acquired another 75 (and 61 and 150v) as part of a package of engines and an airframe I bought from a club member giving up, this 75 has much more compression so I've done an engine swap.

 

So engine out I decided to see if I could see why the compression was down, here's some photos. After 30+ years and many hours running, many probably using castor, the piston rings are still free the bore looks good, but there was some carbon build up on the exhaust valve and the wrist pin nylon pads are a bit worn, can't see anything obvious as to why the compression is down though.

[Jon H]

I think that might be a 61 Frank. The 75 had flats on the side of the cylinder boss and they are missing from yours. 

 

Check the bore, a 61 will be just shy of 24mm and a 75 26.5mm. 

 

Even if it is a 61 its still running slow 😕 

[J D 8]

  What is the diagonal line on the top ring in third pic? 

[Frank Skilbeck]

8 minutes ago, J D 8 said:

  What is the diagonal line on the top ring in third pic? 

I don't know, I only spotted it on the photos.

[J D 8]

         Has the ring cracked along that line? It would be unusual though.

[Frank Skilbeck]

2 hours ago, Jon - Laser Engines said:

I think that might be a 61 Frank. The 75 had flats on the side of the cylinder boss and they are missing from yours. 

 

Check the bore, a 61 will be just shy of 24mm and a 75 26.5mm. 

 

Even if it is a 61 its still running slow 😕 

John, just checked you are correct, I'd always assumed it was a 75 but never measured it. The replacement one I have put in is fatter across the fins and does have some flats on the side of the cylinder boss, I'll have to get the mods to change title!

 

As you say still weak for a 61, if I recall my own old 61 would easily turn a 13 x 6 well above 9,000 rpm.

[Jon H]

35 minutes ago, Frank Skilbeck said:

 

 

As you say still weak for a 61, if I recall my own old 61 would easily turn a 13 x 6 well above 9,000 rpm.

 

CLoser to 10! I have a 61 here somewhere and its very fast

[Frank Skilbeck]

Long while since I checked my 61! Hopefully will see what the replacement 75, I've put in, will do at the weekend.

[Doug Campbell]

Didn't realise they had two piston rings. Good for sealing but poor for passing lubricant to the bottom end?

[Jon H]

6 minutes ago, Doug Campbell said:

Didn't realise they had two piston rings. Good for sealing but poor for passing lubricant to the bottom end?

 

Cant be that poor, its 35-40 years old and it didnt seize up yet 😉 

[Andy Stephenson]

12 minutes ago, Doug Campbell said:

Didn't realise they had two piston rings. Good for sealing but poor for passing lubricant to the bottom end?

The original design of the early Lasers only had 1 piston ring. When my 75 started to loose compression I sent it back to Neil for the conversion to the 2 piston ring version which came in after I bought mine in 1986.

[Doug Campbell]

1 hour ago, Jon - Laser Engines said:

 

Cant be that poor, its 35-40 years old and it didnt seize up yet 😉 

See question mark at end of sentence. I don't know so was asking 

[Frank Skilbeck]

23 hours ago, J D 8 said:

         Has the ring cracked along that line? It would be unusual though.

JD, I looked at the ring more closely, it's not cracked and the line is so perfect it's like it's a design feature.

 

Now how do I get the exhaust valve collets back in without a valve spring compressor 🤔 any suggestions?

 

[Engine Doctor]

That mark on the ring looks like a machine mark. Check the ring gaps . The rings also look quite tight to the piston and probably not pressing on cylinder hard enough . Fit new rings and valve spring and it'll run fine.

[J D 8]

 Hi, Frank, thanks for the info about the line, interesting and odd.

The way I get them pesky little collets into the holder is place head on a flat surface [ if head dished a bit of folded paper to push valve into seat] One hand holds the holder/spring down and I use a small magnetic screwdriver to place the collets.

  All this is done in a clear plastic bag to contain at least one inevitable ping.  John.

[Jon H]

Make a little wooden block that fits inside the combustion chamber so the valves sit straight up. Then use a small offset ring spanner to do the pushing and pop the collects in