Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

[harrylarson]

Have a 1978, 150 hp, Johnson, pulled the heads and found #3 cylinder is deeply scored, looks like a ring came apart. I decided to tear it down and completely rebuild (never did this before but I'm looking forward to it).Then I thought, what if I go to all this trouble and find out that it's scored so deep that it can't rebored, or someone before me already did this and there is'nt enough of the cylinder wall left? Any thoughts?

 

[tpenfield]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Yes, if the cylinder wall gets too thin. . . then there is the issue of pistons that would be available. Typically there is up to 0.030" over-bore on pistons.

A Machine shop should be able to take some measurements and see where you are at. Otherwise it is a new powerhead (ouch)

 

[will62]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Oversized = .020, .030, .040... Look for one of those numbers on the top of your pistons to tell if it has been rebored previously.

Edit: Wiseco for your motor: .020, .030, .044, .064
page 15 - http://www.wiseco.com/Catalogs/Marine/CompleteCatalog.pdf

 

[sschefer]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

If the gouge is too deep to bore you may be able to have it resleeved. Not sure if you can do that on this motor or not. As was mentioned above, the limitation is oversized piston availability and that likely relates directly to what is safe from a cylinder wall thickness perspective. IE., max over size piston might be + .040 so max safe wall thickness might be -.040. It seems like such a small amount in perspective but there's a reason, you can be sure of that.

 

[emdsapmgr]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Factory pistons which will match your original pistons in terms of weight (rotating mass) and thermal expansion can be purchased from Bombardier up to .030 over. Typically it's best to match original pistons with the same type, but in oversize. You can have a machine shop test bore one cyl .030 over and see if it cleans up. Other manufacturers make larger oversize pistons. Best not to mix one of these with your other 5. Go with all 6 from the same manufacturer. Probably best to rejet the carb main jets up a size when going with pistons which are more than .030 oversize. If it's possible to resleeve, you could sleeve it back to original (standard) size. These blocks were made by the thousands over the years. You should be able to find a good used one at a reasonable price from most marine salvage places.

 

[harrylarson]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Thanks all, for the info. I haven't seen any numbers on top of the pistons, I did take off the exhaust cover and the rings on piston #3 don't look any different from the rest. I have attached some pictures (if I did this right), can anyone tell me what caused this damage to the inside of the head and the top of the piston? Thanks.

 

[emdsapmgr]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

The damaged piston got too hot. Either from lack of lubrication (high speed jet is plugged up in the carb) or you have carbon buildup on the piston skirt, preventing the fuel mixture from (reaching) lubricating the piston rings. (If it were an advanced timing issue, you would see problems with all the pistons.) It is also possible that the rubber water diverters is out of place or swollen such that it won't permit water flow around that cylinder liner. Once the rings get hot, one will crack. When it cracks, eventually one sharp edge of the ring will catch on an intake/exhaust port and break that ring into smaller bits. These small bits will get blown into the combustion chamber where they will jam themselves between the piston crown and the under side of the head. A fairly common failure on these crossflows. I'd probably pull that carb apart that fed that cylinder and pull out the high speed and mid jet from that carb throat side. Visually check for any debris in the jet throat. If the jet is clear, pull the intake side (bypass) cover off the powerhead that supplies that cylinder and have a look at the piston skirt-for excessive carbon buildup. If you decide to overhaul this block yourself, you'll want to know just what caused the failure so it won't happen again.

 

[harrylarson]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Thanks emdsapmgr. Yes I plan on rebuilding this motor. I plan on doing it completely, boring all 6 cylinders and rebuilding the carbs. So if I make it that far I should clear up what you mention about the carbs. By the way I could use a little more help about the sequence of the tasks involved in a rebuild. I am assuming that I should get the powerhead to a machine shop first and go from there, right? Thanks for your help.

 

[emdsapmgr]

Re: Can A Cylinder Be Scored So Deep That It Cant Be Rebored?

Look for a marine (type) block machinist. They are most familiar with boring marine blocks and the specific clearances that these two strokes require. Keep in mind it is ok to just overbore one cyl, as long as the oversize piston matches the other 5. You'll want to tear the powerhead down first and visiually inspect everthing-first. I'd pay close attention to the rotating assembly. Make sure the crank journals look pristine. Ditto for the crank seal rings. You don't want any blue needle bearings either. Check out the upper and lower main crank bearings. The upper is really robust, but check them out for pitting. Visually inspect the bridges between the intake and exhaust ports. When a piston overheats to the extent that the cylinder liner scuffs the piston (as yours did) it is always possible one of those bridges cracked due to heat. That will scrap the block, esp if the crack is through the cyl. liner and the aluminium of the bridge of the block. Once you are satisfied everything looks ok, it's off to the machinist.