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Pistons & Sleeves?

dardoonk

dardoonk

Forum Mentor
With no work today & crummy rain, it was a good day to let my wonder a bit... Looking at a couple blocks I have, I started wondering why the sleeves protrude so far thru the block?? I understand they have to be long enough to fully cover piston travel, but does it have to be as long as they are?

One consideration would be to reduce pistons rocking back & forth in the bore, but that only makes sense if the piston skirts are long enough.. Now that skirts are often very short or almost none, there must be some other way engineers reduce the rocking motion of pistons..

With BB kits that require new sleeves, it looks like the four inside studs become the problem when you bore the cases. If the sleeved didn't have to be so deep, or could even be eliminated for a portion of their circumerence (say, the sides where they probably the least needed) maybe bigger 8V GS motors could be built.
 
dardoonk said:
With no work today & crummy rain, it was a good day to let my wonder a bit... Looking at a couple blocks I have, I started wondering why the sleeves protrude so far thru the block?? I understand they have to be long enough to fully cover piston travel, but does it have to be as long as they are?

One consideration would be to reduce pistons rocking back & forth in the bore, but that only makes sense if the piston skirts are long enough.. Now that skirts are often very short or almost none, there must be some other way engineers reduce the rocking motion of pistons..

With BB kits that require new sleeves, it looks like the four inside studs become the problem when you bore the cases. If the sleeved didn't have to be so deep, or could even be eliminated for a portion of their circumerence (say, the sides where they probably the least needed) maybe bigger 8V GS motors could be built.
Click to expand...

You can shorten the new sleeves, which I'm doing on my present BB engine. I leave enough length to fully support the skirt bottoms though.

My pistons are 7 mm shorter than stock, so there is already a greater potential for them to rock. You can scallop the bottom of the sleeves to match the cutaways on your pistons. It makes for easier jug fit ups too, as part of the rings are exposed each side. Make sure you do your sleeve shaping after they have been re-fitted into your cylinders and been bored.

If you get too greedy with bore size on air cooled road bikes, you'll pay the price with poor gasket sealing and overheating. Higher CR's and less ali surrounding the sleeves can hamper efficient heat dissipation.
 
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Hmmm.. I am wondering if I can scallop the sleeves to allow clearance for the inner studs??
I'm afraid the studs will be to weak, unless I come up with a modification of some kind. That's what got me wondering why the sleeves are so long to begin with. My sleeves measure 83.18mm outside for a 78mm bore.
 
dardoonk said:
Hmmm.. I am wondering if I can scallop the sleeves to allow clearance for the inner studs??
I'm afraid the studs will be to weak, unless I come up with a modification of some kind. That's what got me wondering why the sleeves are so long to begin with. My sleeves measure 83.18mm outside for a 78mm bore.
Click to expand...

Kurt, my present 850 project has sleeves with 76 mm bore and 81 mm OD. I wasn't prepared to go larger than that because of the concerns you have mentioned. Are you planning on racing yours? What CR have you settled on?
Are you running the small port or big port 850 head?

The length of sleeves could be trimmed back to level with the cases, but you risk them distorting under load. If you put a radius on the scalloped extremities (passed the sealing surfaces), you could prevent possible cracking in those areas. The only way to tell, is to try it out. Go for it.
 
You can cut away the top lip of the sleeve far enough to clear any studs but you cannot scallop the lower sides of the sleeves for stud clearance.
You also need to leave aluminium between the stud holes in the block and the sleeve as if there's only air behind the sleeve you will have an odd shaped bore when it's hot - and "hot streaks" down the bores.
For a drag motor it may be okay....but not for road use certainly.

Yes, cut away the bottom sides of the sleeves as 49er suggests to match the piston skirt at bdc - always a good thing in multis as it aids under - piston airflow between cylinders.
 
49er said:
Are you planning on racing yours?
Click to expand...
No racing, just a Big Arse street motor.:cool:

49er said:
What CR have you settled on?
Click to expand...
Plan to knock it down to around 9.5. Aside from the inner studs, my concern is heat & I don't need to squeeze every pony out of it.

49er said:
Are you running the small port or big port 850 head?
Click to expand...
I am actually building from a 1000 block & using JE ~78mm (measured) they are stamped 79mm underneath. The head I have is a V&H piece, ported, SS valves, shim under buckets & Andrews 3S cams.
PICT2975.JPG


GregT said:
You also need to leave aluminium between the stud holes in the block and the sleeve as if there's only air behind the sleeve you will have an odd shaped bore when it's hot
Click to expand...
There's still alum there, but not a lot. My concern is leaving enough alum on the block after they are clearanced to keep the studs in place.
 
There's still alum there, but not a lot. My concern is leaving enough alum on the block after they are clearanced to keep the studs in place.[/QUOTE]

Why ? The studs are located by threading into the case at the lower end and by the head at the top...
So long as there's clearance enough to slip the block on you'll be OK.
The block should be located by the dowels on the bottom face, not by the studs.
 
I understand.. Right now, eyeballing it, it looks like I'd be leaving very little on the block where the studs mount. When sliding the 1238 BB onto the case it appears there is very little that has to be removed, especially around the studs, however, they are 80mm OD sleeves & the ones in the block I want to use is 83mm. Guesstimate at this time is I would have to take 1.5mm-2.0mm off the cases where the studs mount.

I don't want the studs pulling from the case, so I thought it could cut away the sleeves in that area, or maybe even reduce the OD of the sleeves where they protrude from the block in order to leave more alum around the studs.

Has anyone run a sleeve as big, or bigger, than the ones I am trying to run on a street motor? Or, at least on a motor that was intended to last more than 15 minutes at the strip?
 
We have some confusing terms being used...as i understand it you're going to have to enlarge the openings in the top case to suit the bigger OD sleeves - correct? And you're worried about the amount of material left around the stud boss in the top case.
At this point all you can do is mock up the pistons/rods/crank and see where the piston side cutaways fall in relation to the inner stud positions.
It may well be possible to leave material around the stud bosses - awkward to machine but not impossible.
The cutaways in the side of the sleeve can go up to just below the bottom ring position at BDC.
 
I wouldn't worry about the studs pulling out. As Greg said, the block is located by spigots, so it can't move sideways, even if the studs try to move under torque/or expansion. Besides, the area you are concerned with will be supported on that side by the sleeve.

Are you fitting O/S studs? I won't be. My present 850 at 10.5-1 is running a 5 layer MLS head gasket and torqued to the stock figure of 30 ft lbs. No problems.
I am running a slight protrusion of the liners though, which increases the bite around the fire ring.;)
 
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GregT said:
We have some confusing terms being used...as i understand it you're going to have to enlarge the openings in the top case to suit the bigger OD sleeves - correct?
Click to expand...
Yes, correct.
GregT said:
And you're worried about the amount of material left around the stud boss in the top case.
Click to expand...
Correct, again.
GregT said:
The cutaways in the side of the sleeve can go up to just below the bottom ring position at BDC.
Click to expand...
I won't be able to mock anything up until next weekend, but here's what I know right now...
On the stock 1000 motor, when the piston is at BDC, the bottom ring sets 7mm above top of the case. The crankpin sets 3-4mm below the top of the case (same for the new piston).
The bottom of the new piston is 27mm below the center of the crankpin in the front & back and tapers to 14.5mm below the crankpin on the sides.
So, I think that I understand I could cut the sleeves in the same shape as the piston skirt as long as I can fully support them?? If so, my sleeves would extend approx 31mm beyond the block (front & rear) to a minimum of 18mm on the sides (assuming the cuts on the sleeves mirror the piston skirt).

Let me know if I have this all mixed up, but I'll do some closer inspection next week.
 
I don't have a 1000 apart at present to check but memory says the stud boss is quite deep - 20 - 25 mm ?
If so you don't have to machine out more than you need for sleeve clearance - I mean depth from the top here. A compromise in sleeve length on the sides and maybe a cut about 8mm down the bosses will leave plenty of material left to hold the stud.
The front and rear thrust faces should still be long enough to cover the piston skirt at BDC.
Again still difficult machining but not impossible.
 
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