Cylinder / Piston help

[sauldgold]

Hi all,

Please bear with me, this is my first engine rebuild, so my questions may sound dumb to some.

I've searched the threads, but haven't found my answer.  I have a '78 G5 with 1000cc, iron cylinders and 60k miles.

Compression was bad - 120 and 105.

I pulled the heads and my next step is a trip to the machine shop to measure the cylinders to see if they are within spec and can be rehoned and retained.

However, I want to get an idea of next steps if they are not salvageable.

1. Can they be slightly bored and used with slightly bigger rings on the original pistons and still function properly?

2. If that is not possible and / or a dumb idea, are 1st over pistons available and / or advisable ? (they don't seem to be for sale from any of the retailers, so I'm guessing no, but want to confirm)

3. If the answer to both of the above is no, is the only option to buy two complete cylinder, piston, ring kits?

Thanks in advance.

[cliffrod]

If you bore a cylinder, you cannot use the existing piston with larger diameter rings.  You must install a larger diameter piston with matching rings to maintain appropriate clearances for operation. 

Honing a cylinder and replacing rings while using the original piston can be an option.   It works best when there aren't deep pits, erosions or  scratches in cylinder walls. 

[sauldgold]

thank you cliffrod. 

if it comes to that, are over size pistons and rings even available for the 1000? 

so far i have only seen oem spec cylinder/piston/ring kits sold as a package.   is that just the standard course of action in such situations ?

[Antietam Classic Cycle]

I've never seen oversized pistons sold anywhere, you could always adapt VW Beetle pistons. 

Really though, IMO your best bet is just pony up the $$ and buy Gilardoni kits.

[cliffrod]

Do what Charlie says.  The job will be properly addressed, people will be able to see its been done from the Gilardoni name on the outside of the cylinders and this will help the bike to be easier to sell someday in the future.

Custom pistons and rings are more readily available now than ever.  But people that know these engines would rather see Gilardoni cylinders than a workaround explanation & receipts.....

[normzone]

[sauldgold], I think the above posters are wise in their counsel.

Everything I know from my limited experience in years gone by indicates that the oversize pistons/rings/barrels world was based on older machine tech that was designed back when things had to last a long time and were expected to be rebuilt once or more.

Today the performance expectations are higher and the longevity expectations are shorter.

Sad but true, but fortunately good tech is available to compensate.

[s1120]

granted not having measured I could be wrong... But those cyl's LOOK like they are beyond just a hone and re ring.  I didn't know about the lack of oversize, but I guess it makes since being there are a few different types of cyl wall construction on these, and some can, and cant be board, and would need deferent pistons if it could...  Really by the time you got new pistons, rings, and machine labor costs... your looking about the same price as the replacement cyl's anyways... 

[Mike Tashjian]

Compression was low.  First question I always have is how did you do your test?  Did you do a leak down test?  I know many people jump into projects without doing the basic testing properly.  Mike

[Chuck in Indiana]

granted not having measured I could be wrong... But those cyl's LOOK like they are beyond just a hone and re ring.  I didn't know about the lack of oversize, but I guess it makes since being there are a few different types of cyl wall construction on these, and some can, and cant be board, and would need deferent pistons if it could...  Really by the time you got new pistons, rings, and machine labor costs... your looking about the same price as the replacement cyl's anyways...

Yeah, without seeing it, it appears to have rust pits that won't clean up with a hone. Now, if I were in Alaska and needed to get home, I'd hone the crap out of it, put in new rings and head out..
It would do it with ease.. 

[guzzisteve]

Well, looky here---- Need 1more

https://www.ebay.com/itm/MOTO-GUZZI-1806-0350-NOS-PISTON-WITH-RINGS-1st-OVER-40mm-1000-CONVERT-SP-G5/182763030676?epid=2241481332&hash=item2a8d868894:g:nKcAAOSw9LxZuWOn&vxp=mtr

[s1120]

Yeah, without seeing it, it appears to have rust pits that won't clean up with a hone. Now, if I were in Alaska and needed to get home, I'd hone the crap out of it, put in new rings and head out..
It would do it with ease.. 

Oh without a doupt!   Ive done many driveway ring slaps in my day!  And some ran for quite some time..

[pete roper]

[sauldgold], I think the above posters are wise in their counsel.

Everything I know from my limited experience in years gone by indicates that the oversize pistons/rings/barrels world was based on older machine tech that was designed back when things had to last a long time and were expected to be rebuilt once or more.

Today the performance expectations are higher and the longevity expectations are shorter.

Sad but true, but fortunately good tech is available to compensate.

In fact Nicasil is superior in every conceivable way compared to a cast iron liner and will last pretty much indefinitely. The suggestion that it was chosen because it wasn't expected that bikes thus equipped would be rebuilt is bunkum. It's just a far, far superior solution to sealing a piston in a bore.

Pete

[Furbo]

Regardless of which direction you go on your cylinders, dissassemble your valve gear and you'll likely find pitted seats and worn guides. It may well be the primary cause of your compression issues. It's my experience that when guzzis have compression loss because of piston/cyl fit, they blow oil out the crank breather. I've had two bikes that did that. I literally had to wire a small water bottle to the frame and run the breather hose into it. Every gas stop I'd pour the oil back into the engine. I did eventually get new cylinders for both bikes. (and did valve job while the heads were off).

Guides are readily available and any competent machine shop can do the valve job. A shop that deals with AC VW's is a good bet.

[moto-uno]

 I too have concerns about the compression , it's definitely low , BUT , did you take it with the throttles wide open ? You can't compress what's not there :).
 Before this comp test did you check that there was some valve clearance ?
 Have you (or a machinist ) checked the piston clearance ? You haven't posted a follow up on this.
 It appears from the 1500 miles per year usage ( 60k /40yrs), do you want to start spending that kind of money ?
 Just asking,  Peter

[Turin]

I think I may have a pair of nos cylinders in my garage.
I can look Sunday when I get home.

[LeRoy]

Gilardoni cylinders and pistons will run you up to $1000 when you include the assortment of gaskets and related consumables that you'll need. You never hear anyone say that they regret the cost when it comes to building a Guzzi engine that will run and run and run. But frankly, $1000 is a big chunk of what a decent G5 project might cost.

So, if you are budget-constrained, or a bottom-feeder (I am; Guzzi content), or you simply want to spread your money over additional priorities, then you might want to consider patiently watching eBay and other sources for a set of used cylinders and pistons from a V1000 G5 or Convert. Shop wisely and patiently and you may find a perfectly serviceable set for modest money.

It all depends on what you want out of your G5. If you're going for a nut-and-bolt perfect restoration, then used parts probably aren't part of your plan. But if you want a serviceable bike with plenty of life and miles in its future, then a good set of used pistons and cylinders can be a worthwhile expenditure that also leaves more of your budget for other aspects of putting the bike back on the road.

You'll obviously want to look carefully at any parts you're considering. If you don't feel confident in buying this sort of used parts, then your best bet will be the new stuff.

Best of luck with your project.

By the way, if you'd like a CD with the full gamut of V1000 G5 factory manuals (parts, shop and owner's manuals), repair best practices, wiring diagrams, period road tests and articles, brochures, and more, then send me a PM with your full name and mailing address. I'll send you the CD at no cost, anywhere in the world. I also have model-specific CDs for most Tonti-frame Guzzi models from the mod-1970s to the early 1990s. This offer is open to all WildGuzzi members. Just contact me by PM with your the model you'd like and your mailing information. I'll send the CD(s) you'd like. No cost. Sent anywhere in the world.

LeRoy