Tue, 08/24/2010 - 08:02
Hi all,
Any and all help on this appreciated. Problem: piston stuck in a long time sitting R27. Want to un-stick. Was told not siesed, so must be rusted. (hope)What I see: have head off, piston a little below mid stroke (41mm down). Cylinder wall lightly surface rusted, no scoring or nasty trauma apparent. removed cylinder bolts and can move cylinder/piston up and down, kick start will move them also, so appears tranny, crank, cam etc is free and smooth. What I have tried: PB blaster sitting in cylinder for 3 days. Acetone/ATF cocktail sitting in cylinder for 3 days. Slight heat from torch. Using a hard palstic pipe that meets at the edge of the piston and hitting with hammer, but not awful hard.
So, where to go from here? Can the con rod be removed at the crank if I go into the case, so I can remove the cylinder/piston/con rod and get at the bottom of the piston? Get more aggressive with the hammer? Get more aggressive with heat. Use shop press?
I have resigned myself to replacing the piston but want to save all else.
Thanks in advance for any advice.
Larry
- 635 reads
Tue, 08/24/2010 - 08:36
#2
Well, you're tried quite the
Well, you're tried quite the range of penetrants. I would have suggested AeroKroil, but that Acetone/AFT cocktail is supposed to be even better. I'd be inclined to let time do its thing, maybe wait another 3-4 days. Heat is a good thing, too, to help the penetrant work deeper.
I don't know if the conrod can be removed from the crank. I know it can on the /5-on bikes...takes a special bit to get at the screws. Can you see down to the bottom of the conrod to find a couple of sunk bolts holding it on?
Something else that might be considered is to put together something to adapt to the head bolt holes and create a pusher...not a puller. If you can get something to screw down onto the top of the piston to apply constant pressure...the head bolts would react the load and keep the plate/adapter attached. Maybe with constant pressure, penetrant, the occasional heat blast, it will let loose.
Tue, 08/24/2010 - 09:33
#3
Yeah, I agree, an over bore
Yeah, I agree, an over bore is probably in the cards,and pistons seem quite available. Its breaking/cracking the cylinder or doing other damage to the lower end the concerns me. Heat cycling looks like the next, easiest plan. When working on really stuck bolt I will sometimes heat it up then toss cold water on it. Might heat the outside of the cylinder then pour cold water on top of the piston. Any opinions on how hearty the cylinder is to heat/cooling? Also, as it will need some encouragement with a hammer, is my belief that as the piston is 41mm down from the top edge of the cylinder and it has a 68mm stroke it still has downward movement to go? Don't want to just be beating on the crank at BDC.
Thanks for the replys.
Larry
Tue, 08/24/2010 - 09:47
#4
Sounds like you have it sussed...
Although the cylinder can take getting splashed with cold water while the engine is running, I would still do my heating on the piston. If you try to heat the cylinder with some kind of torch, I think you're going to get dramatic differential heating at different places on the cylinder. You could heat it in an oven, though.
Kurt (or someone else with experience with the singles) will have to tell you about the piston stroke. On the twins, you would certainly be right that the piston is only part way down the bore, and I believe but cannot confirm that you are right about your single.
Tue, 08/24/2010 - 09:55
#5
It does appear that the R27
It does appear that the R27 stroke is 68mm:
http://www.bmbikes.co.uk/specpages/R27.htm
I don't know if the piston comes all the way to the top of the cylinder wall, but pretty close I'm sure. So, you are certainly at or just below mid stroke.
Tue, 08/24/2010 - 10:18
#6
So, heat the top of the
So, heat the top of the piston to ensure even heating. Makes sense. Since I've pretty much planned on replacing the piston I could hit it with quite a bit of heat I think.
Tue, 08/24/2010 - 10:27
#7
I'd be careful of too much
I'd be careful of too much heat to damage the cylinder walls.
The thing you're trying to do is get some separation from the walls. Heating basically expands everything...aluminum will expand faster than the steel. What about getting some dry ice and placing some chunks on top of the pistion, maybe using some kind of plastic PVC to help insulate the dry ice from touching the cylinder walls. If you could get the piston to contract faster than the cylinder walls cool down, you might create some separation.
I'm sure dry ice is available in most bigger cities...just have to ask around.
Tue, 08/24/2010 - 10:39
#8
Curious idea--if anything dry
Curious idea--if anything dry ice is fun to play with. So, help me out. What are these components made of--there it the piston, the cylinder and, I think, a sleeve in the piston. Correct? What is aluminum, steel or iron?
Continued thanks.
Larry
Tue, 08/24/2010 - 11:03
#9
The cylinder is cast iron.
The cylinder is cast iron. The rings are iron, steel or chrome/steel. The piston is aluminum with perhaps steel inserts.
The dry ice idea sounds good. You're trying to get the rings to move around and break whatever bonds the rust has created between the rings and the cylinder walls. If you get the pistons to expand a lot, they will tighten up on the rings, forcing them outward. When they cool, the spring pressure in the rings should make them shrink back. The rings won't expand or contract nearly as much as the piston, thermally. But if you can get the piston to shrink quickly, maybe the rings will >snap< back and break free.
Wed, 08/25/2010 - 08:00
#10
Update. The piston is out. It
Update. The piston is out. It never really broke free--had to be banged out the whole way. The heat cycling I think got it started. Cylinder is in not too bad shape. Rings are very frozen on the piston. Thanks for your help. I will be back, I'm sure, as I try to build this bike. Next is trying to figure out the engine electrics.
Cheers,
Larry
Wed, 08/25/2010 - 08:21
#11
Feel the surface of the
Feel the surface of the cylinder -- you can tell more with your fingers than by looking. If you can catch your fingernail on any surface imperfection, you're going to have to at least hone until that's gone. Perhaps you can find a replacement piston and rings that will match your cylinder after that. Most likely, however, you will have to bore; buy the piston and rings first and bore to match them.
The electrics on the single are different from the twins. The singles use a battery/coil (Kettering system) ignition; they won't run without a good battery, and they won't run long unless the generator is working correctly.
Wed, 08/25/2010 - 08:21
#12
Larry -
Great! Always good
Larry -
Great! Always good to move on to the next step.
Vech sells a book by Doug Rinckes which provides a lot of details of the engine electrics, charging, ignition, wiring, etc. There's a free version out on the web, but the updated version is worth it.
Wed, 08/25/2010 - 11:58
#13
I'm planning on the over
I'm planning on the over bore. Any reccomendations for who to send the cylider to and they can both sell me a piston and bore for it correctly? I'll start looking locally first. Should I go with a vintage BMW knowlegable shop or is a bore job something my local machine shop can handle if I hand them the piston.
Wed, 08/25/2010 - 14:54
#14
Take a look at Bore-Tech.
Take a look at Bore-Tech. Carbide impregnation...like Nikasil for the '81-on bikes.
Sat, 08/28/2010 - 14:50
#15
New to the group so to late
New to the group so to late for this time - but maybe helpful for the next.
The soft way:
1) break the ceramic isolation of a spark plug
2) Try to solder or weld a grease fitting to the plug.
3) Fill the cylinders upper side with grease.
4) Screw the plug in the head and fit it on your cylinder - open the valve adjusting screws (so valves should be closed)
5) Attach a grease press and press the piston out.
Regards
Axel
Sun, 08/29/2010 - 08:32
#16
Just wondering out loud
I'm pretty new to this group and to these vintage bikes so maybe I'm off base with this, but I've seen quite a few posts on spark plugs being blown out of the heads. If you use the spark plug hole as an injector point for a grease press, and given that there is already a tendancy for these plugs to come out, I'd think you run the risk of stipping out the threads, adding to your already high frustration level. If my math was right, there would be something like 36 times as much force pushing on the sparkplug as on the piston assuming a R60 with a 72 mm bore and a 12 mm sparkplug hole (that size on the sparkplug hole was just a guess - my bike is still in transit). I freely admit it has been, well, decades since I've done any actual engineering work so I might have the math wrong, but thought I'd mention this as a caution point.
Sun, 08/29/2010 - 08:39
#17
Instead of grease, fill the
Instead of grease, fill the hole with rope through the spark plug hole and then use the kick starter or better yet, put the bike in gear and try rolling the bike back and forth to apply pressure to the stuck piston.
Sun, 08/29/2010 - 11:33
#18
Spark plugs and rope
The spark plugs come out of the earlier heads when the insert they're screwed into lets go. This generally happens to mid 60s "butterheads", which are made from an unstable alloy*. Earlier heads and the later LK heads don't lose spark plugs, as a rule.
The rope trick** is apparently useful for getting a stuck head off the cylinder, but if your piston is stuck, you won't be able to use the kickstarter.
*) You know you have a butterhead if the head bolts have to be unscrewed to get them out of the head; if the steel stanchions the rocker blocks sit on sink in and take up all your rocker adjustment and eventually require putting washers under the rockers; and if your spark plug insert comes out of the head.
**) I've often wanted to say that, but didn't have anything else to add. 
The conrod can't be removed at the crank, it's a one piece item and the crank is pressed together.
I've not had to deal with this problem myself (knock wood). You could put significant heat into the piston, after all, there are very high temperature explosions happening against it when the bike is running. Heat cycling might loosen things up to the point that the rust lets go. This is probably the safest way to continue.
I have read a number of people putting a block of wood against the piston and hitting it pretty hard. Your cylinder may already have significant rust pits where the rings are, so you may be in for an over bore anyway.
VBMWMO Webmaster,--Darryl Richman
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