88mm Iron bores instead of Gilardonis?

[Turin]

I'm going to have to replace the chrome bores on my 850T. I acquired a pile of spare parts, including a unused set of 88mm convert/sp1000/G5 cylinders.

My thought is to use these cylinders with some high compression 88mm pistons. ( the bike already has a cam and 36mm carbs)

Please tell me why this won't work, or why it will be such a hassle that it wouldn't be worth it.

Thanks!

[LeRoy]

I may be mistaken but I thought the barrel spigots into the block were smaller diameter on the 850-T. If so, you'd need to open up the spigot diameters in the block or turn down the diameter on the barrels. That doesn't mean it won't work. It's just that the amount of labor and machining to make it work might not be worth the effort. On the other hand, if I'm wrong, someone is sure to come up with the correct information.

[EldoMike]

Pretty easy to get the spigots turned down...can't remember the number but it wasn't that much.

[pressureangle]

On anything less than hard usage, there is no difference between an iron liner and aluminum.
I'd say since you already own them, use the iron. Make sure you consider piston ring compatibility.

[John A]

Especially if they are a factory part I would assume with good quality control I'd sure use them. If I remember correctly where iron liners gave trouble was poor fit to the aluminum cylinders.

[Turin]

Fitting the later lemans 1000 Hi-comp pistons with small valve heads in cast iron bores is what I'm curious about, and if there is enough material in the stock big valve pistons for machining recesses for the valves if necessary.

Turning down the spigots doesn't worry me.

[guzzista]

Interesting project . I have pretty much the same ingredients for a round fin motor project, with the exception of the small valve heads having been modified into mid valve size (LM 1, 2,3).  From what I see the  LM 1000 pistons will need some mods  (if doable), but I don't have  anything mocked up to  measure anything. As a side note, the last time I did something similar, G5 pistons, barrels  ( turned down to fit ) and squish band adjustment worked very well on a T3 with small valve heads.

[John A]

I remember doing this in a dark basement and I think there is enough material on the piston. The guy who could answer  is Mike Rich at Mike Rich Motorsports. the problem I had was too much compression and not enough squish . I tried to add a link but its not working for me today

[Rough Edge racing]

  I believe a 950 small port head with 10-1 compression , tight quench  and the proper cam would run on pump gas and make great power.The existing ports will likely need the floors raised where they curve to valve...The half dozen or so custom forged piston manufactures in the USA will make you any piston you want if it matches the forged slugs they have available..You can make a plaster cast of the head combustion chamber so they can machine a lower flatter tight fitting dome..Not cheap, you may have to have them do a four piston minimum, 150 bucks each plus rings...
 This is exactly what makes my 650 Triumph a winner........Just saying

[guzzista]

[quote author=Rough Edge .The half dozen or so custom forged piston manufactures in the USA will make you any piston you want if it matches the forged slugs they have available..You can make a plaster cast of the head combustion chamber so they can machine a lower flatter tight fitting dome..Not cheap, you may have to have them do a four piston minimum, 150 bucks each plus rings...
 This is exactly what makes my 650 Triumph a winner........Just saying
[/quote].  This is when the Gilardonis start to gain in terms of convenience / cost advantage

[pete roper]

If you try and use any of the readily available pistons for mid or big valve engines with small valve heads you will have valve to piston clearance problems with the small valve heads and the compression will be too high for gas that met 25-30 year old octane levels never mind the E-10 cats piss you blokes have to live with nowadays.

The only �Factory� piston configuration that might come even remotely close to working, (And I�ve never done the sums.) would be the SPIII ones and even then my guess is that they might need relieving for the valves to avoid valve/piston contact on overlap.

Another thing is while increasing compression will, in theory, give you �Free Performance� the reality is that as a rule the extra weight may require rebalancing of the crank and the alp of alloy on the top of the piston will bugger up the combustion chamber and can lead to all sorts of nasty end-gas issues.

Is it doable? Yes, most things are, but it will be neither easy or cheap. In reality Nicasil is a huge improvement over cast iron as a bore material. As you know a small valve engine with a slightly warmer cam and bigger carburettors with the heads ported to suit is a lovely thing. You can get noticeably more torque by going up from 83 to 88mm in the bore but outright power is unlikely to be improved noticeably. My advice would simply be to go with a stock SP/G5 set of Gillardonis and if you want a bit more grunt get a bit of head work done and leave it at that, perhaps if you�re keen twin plug it. Other than that stuff will start becoming a time consuming example of the rule of diminishing returns.

Pete

[blackcat]

“As you know a small valve engine with a slightly warmer cam and bigger carburettors with the heads ported to suit is a lovely thing.“

This.

The only thing that I would consider is a lightened flywheel. I have the above with the lightened flywheel on my CX, and I wouldn’t bother with anything else.

[Turin]

I thought since I already had the barrels, that maybe there might be a way to do this on the cheap.

Back to Plan A - Gilardonis with copper head gaskets.


 Anyone need a set of Iron bore barrels?

[LeRoy]

Back to Plan A - Gilardonis with copper head gaskets.

Will you be going with the 850cc Gilardoni set or the 949cc "big bore" set they make with turned down barrels just for the 850 engines? This gives you the added displacement for just a small bump in price over the 850cc kit.

https://www.mgcycle.com/index.php?main_page=product_info&cPath=68_129&products_id=2131&zenid=9c14b5713e30b79cb7cee5c9fba9ec9a

[Turin]

For laziness and cheapness sake I'll keep her as an 850. Maybe I'll throw the Iron barrels on e-bay. I hate to hoard parts.