New 750 pistons

TeddyG

TeddyG

Hero member
Location
Portland Oregon
The cylinders were badly scored and out of true, so a re-bore and new .5 over Ross forged pistons from Wolfgang have been installed. The valves also needed work, so hopefully this 750 will perform as it should. I'll post the results once it's running again
 

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Thanks for all your continued help. The machinist said that the valves were not sealing well and most likely responsible for the poor engine performance. I'm eager to get this beautiful bike running the way it should. Next will be upgrading the brakes and suspension, and I'll have the perfect bike!
 
I'm taking no chances and following how the machinist who bored the cylinders labeled them. Wolfgang was right when he told me to first insert the pistons into the cylinder which eliminated struggling w/ ring compressors- an easy install. Another mechanic said he likes inserting the pistons w the engine laying sideways.
 
TeddyG said:
I'm taking no chances and following how the machinist who bored the cylinders labeled them. Wolfgang was right when he told me to first insert the pistons into the cylinder which eliminated struggling w/ ring compressors- an easy install. Another mechanic said he likes inserting the pistons w the engine laying sideways.
Click to expand...
Are they Deves piston rings? I got some from Wolfgang too, and they are the only rings I have ever encountered on a Lav that required fitting the piston from the top, otherwise it was not possible to fit the thin oil rings.
 
When you insert the pistons from top of cylinder, and then have to lower cylinders onto crank, how do you go about fitting the gudgeon pins? Am curious as it sounds like a great idea if gudgeon fitting straight forward.
 
Actually, I found that inserting the pistons into the bottom of the cylinder worked better and the rings just compressed by hand. With the pistons hanging down just enough to reveal the gudgeon pin holes, I carefully lowered the cylinder and aligned the piston hole to the connecting rod hole, and inserted the pins w/ a little bit of giggling (having first inserted the retaining clips on the inner pistons). I used a block of wood in the center to keep everything at the right height and I also made sure the rods were near TDC. I found this much easier than using ring compressors but I'm sure others prefer the traditional method.
 
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I think Tippie's issues related to the fineness of the oil rings and the taper interfering with insertion.

The concept of fitting pistons to barrels first is an interesting one - I've never had a prob doing it the 'other' way, but fitting to barrels first would be a tad trickier with a 270 crank - and I'm not sure it could be done as the small ends might be too low to allow room to get the circlips in. Might investigate if I ever have to pull the slugs out again.
 
Not many pistons have the gudgeon an easy sliding fit, many need a warm piston and even then the pin can be grabbing. It could make it tricky lining up the small end and getting the pin to enter cleanly. Never had a problem before these Deves rings, and yes it was the extra thin oil scrapers not entering due to the chamfer.
The 90 deg crank throw has not caused any problem.
 
I have only ever (attempted) to install one set of Omega pistons into a triple, it was a 1000. On lowering the cylinders over the completed pistons, I heard/felt a ring crack, this meant the whole plot had to come off and one of the two outer top piston rings had broken, I was using home made compressors which I have used hundreds of times previously without aggro, but never with these very thin top rings. Not having a spare ring the job was aborted. I later pondered on this, and the thought that came into my head was that the very thin top ring had snagged in the taper at the bottom of the bore, and instead of being compressed by the taper had got caught and was loaded sideways, breaking the ring, the ring was still a complete circle but had lost a "segment" the thinness of the ring being the main contrubutary factor it might be better to not have the taper at all (as with the top edge) since the ring compressor keeps everything under control, but when the rings enter the taper of the bore they open up at first and then are compressed by the reducing diameter as the piston enters the bore. Since then I have not fitted any pistons with very thin top rings, and am using up old stock bought many years ago, but I now install them to the top of the cylinder and then push them to the bottom just enough to expose the gudgeon (wrist) pins and install those plus the last three circlips, the first three having been done outside the engine, with the motor laid forward on wooden blocks on the bench, you are not fighting gravity, I have found this works very well, I wish I had thought of it years ago, but as yet I havent proven it with those very thin top rings (0.75mm are they?)

just my thoughts which might help or hinder someone
CLEM
 
Ross pistons I use have 1.0mm top ring, 1.2mm 2nd ring, 3mm oil rings, never had an issue with them
When I bore and hone the barrels to size I machine new ring leads into the bottom of the Liners, as every machine shop doing this shit should also do, is standard practice

I then install the pistons on to the Conrods, install the ring compressors and then barrels next
Never had an issue
 
I’ve only fitted pistons and rings to a standard 750, and did it the traditional way - pistons on rods, then barrel. This was many years ago, using ring compressors and the rings will not have been the thin ones. What I do recall is that in order to get the gudgeon pins in, I had to put them in the freezer and warm the piston. That’s what led to me question about how to fir the gudgeon pin if the pistons are already in the barrels and make warming Ricky, I guess you could use a hairdryer.
 
using Ross pistons the Gudgeon pins are a slide fit, no issues at all
been using them for around 20 years now, cannot comment on the other piston brands used for Laverda engines
never used them, always found most of them to be too heavy
 
SMLAV,
According to Wolfgang, OEM Mondial pistons are scarce and he gave the forged Ross Racing pistons a good review. Hopefully mine will do the trick. Measure your piston ring grooves and compare to the Green Book specs. I would give you my old pistons but I'm stateside.
 
redax5 said:
using Ross pistons the Gudgeon pins are a slide fit, no issues at all
been using them for around 20 years now, cannot comment on the other piston brands used for Laverda engines
never used them, always found most of them to be too heavy
Click to expand...
Hi Redax
What Will you use for a: Laverda 750 to 850 ccm ?
 
sfc vintage said:
Hi Redax
What Will you use for a: Laverda 750 to 850 ccm ?
Click to expand...
I have custom made Ross pistons in stock for 87.0mm, 87.5mm and 88.0mm diameter, far lighter than the Asso pistons
also have oversize custom Liners in stock from LA Sleeves
 
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