750 SF2 engine compression numbers

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Greetings,
After 2 compression tests, one cylinder shows 150 lbs and the other shows 125 lbs. After squirting in a little oil the 125 lb cylinder increases to 150 lbs, which I’m assuming indicates piston and rings issues. So what are ideal compression numbers for these motors, and does 125 lbs compression warrant engine rebuilding? While no rocket ship the motor seems to be performing well and will do 80 mph at @ 5K RPM, but I haven’t pushed it any farther.
Thank you
 
If it's not blowing smoke, burning oil, fouling the plug, starts readily, and you are happy with it's performance then don't sweat it. If and when the time comes, worry about it then. Ride and enjoy!
 
TeddyG said:
Greetings,
After 2 compression tests, one cylinder shows 150 lbs and the other shows 125 lbs. After squirting in a little oil the 125 lb cylinder increases to 150 lbs, which I’m assuming indicates piston and rings issues. So what are ideal compression numbers for these motors, and does 125 lbs compression warrant engine rebuilding? While no rocket ship the motor seems to be performing well and will do 80 mph at @ 5K RPM, but I haven’t pushed it any farther.
Thank you
Click to expand...

Chances are it’s been like that for 40 years.


A rough rule of thumb: If it stops pulling wheelies downhill at 60mph in second gear, you know it’s time to freshen it up. 🤣
 
I was once told the time to fix them is when lumps of piston start hitting the spark plug, and its gap closes. The funny thing was, he was right.
 
Thanks guys,
I bought the bike w 15k miles and it had been abused by the PO and barely held an idle and performed poorly. The cylinder walls were scarred and a local machinist experienced w vintage bikes re- bored the out of true cylinders for new Ross Racing .5 over forged pistons and rings, along with re-seating the valves. An initial compression check showed this discrepancy and I was hoping it would improve after breaking in. The motor is performing much better, but not being familiar w vintage bikes or Laverdas, I’m not sure if it’s performing as it should since I,ve read these supposedly can cruise at 90 mph all day long. But I,m very happy w the bikes handling and vintage vibe and it gets constant compliments like “beautiful bike- what is it?”
 
TeddyG said:
Thanks guys,
I bought the bike w 15k miles and it had been abused by the PO and barely held an idle and performed poorly. The cylinder walls were scarred and a local machinist experienced w vintage bikes re- bored the out of true cylinders for new Ross Racing .5 over forged pistons and rings, along with re-seating the valves. An initial compression check showed this discrepancy and I was hoping it would improve after breaking in. The motor is performing much better, but not being familiar w vintage bikes or Laverdas, I’m not sure if it’s performing as it should since I,ve read these supposedly can cruise at 90 mph all day long. But I,m very happy w the bikes handling and vintage vibe and it gets constant compliments like “beautiful bike- what is it?”
Click to expand...
Don't believe what you read.
Cut off date for vintage period is 31/12/1931
How many miles have you put on the engine since rebuild?
Paul
 
Thanks Paul,

I’ve ridden it @ 2k miles and followed the mechanics advice to vary the speed w/o nearing redline for @ 300 miles followed by an oil change, but perhaps I didn’t push it enough. But the motor seems to be responding well and I don’t have a bench mark to compare it to since I’ve owned only modern super bikes.
 
TeddyG said:
I,ve read these supposedly can cruise at 90 mph all day long.
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True that the motors are robust but I think you'd find yourself exhausted if you attempted to cruise at 90mph all day long on the SF2.

I and the SF2 were much happier at 75'ish when covering all-day distances with periodic 75-85 but not sustained.

All day and multi-day rides in recent years have been on a GTL where I and it seem happiest in the 65-75 range.

The twins are really not interstate or super-highway missiles, much more fun on the country roads.
 
redax5 said:
using mineral oil to run the engine in ?
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Yes, I used 20/50 Bel Ray mineral oil and changed it at 300 miles, and changed it again at 1300 miles with no visible swirl. Possibly I babied it too much during the break in, but varied the speeds without redlining it. I would’ve hired you if not for the distance. Do you recommend mineral oil or synthetic?
 
TeddyG said:
Yes, I used 20/50 Bel Ray mineral oil and changed it at 300 miles, and changed it again at 1300 miles with no visible swirl. Possibly I babied it too much during the break in, but varied the speeds without redlining it. I would’ve hired you if not for the distance. Do you recommend mineral oil or synthetic?
Click to expand...
stay with mineral oil, 20W/50 is good for our old clunkers
I use Liqi Moly 20W50 mineral oil in the workshop on customers bikes and Lucas 20W50 mineral racing oil in my race engines
 
TeddyG said:
Thanks Paul,

I’ve ridden it @ 2k miles and followed the mechanics advice to vary the speed w/o nearing redline for @ 300 miles followed by an oil change, but perhaps I didn’t push it enough. But the motor seems to be responding well and I don’t have a bench mark to compare it to since I’ve owned only modern super bikes.
Click to expand...
Just keep riding it if it runs well and enjoy it.
Take the compressions again in 30 years.
Paul
 
Thanks again guys,

I’ve rarely taken it above 5k RPM, so I might push it a bit more occasionally, but no redlining since that’s pointless and I keep checking the B8ES plugs which are burning nice and clean
 
It's more about load than RPM. I never 'lug' a new engine at low revs, but don't shy away whatsoever from using plenty of throttle to aid bedding in new rings. You don't need to redline the motor, but you definitely don't need to pussy foot either. Basically, I just ride the same way I do for 95% of my riding - the other 5% is WOT :cool: We're not talking 1936 Packards or Model Ts here.
 
Thanks Piranha and an update:
A boroscope shows the nice cross hatching from the rebore, but there’s a series of @1” wide black wear marks/vertical scratches that are on both cylinders, aligning with the front and back sections of the pistons that have indents on the top.
During assembly I staggered the open gaps of the rings and the machinist fitted the rings and attached them to the pistons, and I’m assuming he didn’t flop them.
A mechanic friend will look at the boroscope and give me advice on whether to leave as is, or take the motor apart for hopefully just a rehoning.
So any thoughts on what could be causing these marks- I’ll post pics shortly, thanks
 
It's difficult to clearly see the marks from this boroscope, but they are on the front and back sides of both cylinders. A mechanic friend looked w the scope and feels that some of the scratches are too deep for a rehoning, so now larger pistons and a rebore will probably be needed. Since the marks and scratches are fore and aft, it seems to indicate that the clearances were too tight once the forged pistons expanded w heat, and began binding as the pistons slightly rotate on the pins during operating cycles.
Again, new .5 over Ross Racing forged pistons w new rings, pins, and circlips were installed, and the engine has run @ 2K miles.
So now I have to find a better machinist stateside. Wolfgang recommended the Ross racing pistons but what do others use?
Thank you
 

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From what I can see those marks should clean up with honing - and increase your clearance (although honing would take forever to increase clearance to any degree) - f they are indeed deemed too tight. But I'm no expert in reading bores. Ross Racing pistons in my 880 hi-comp and no problems whatsoever. Pretty sure Redax use them almost exclusively.

FWIW my reco'd SF1 motor gave 195psi left and right way back when.
 
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