[Trials]

Lay you odds that the connecting rod bent because your oil pressure failed. What happened is; the connecting rod got tight on the bearings and the crankshaft forced it to rotate anyways. It didn't bend like that for no reason.

 

[SemiFast]

I opened the engine and transmission. Everything looks ok, I still have to take some measurement to check if everything is up to specs. Decided to clean the engine and polish the case. The only problem is that I cannot find a replacement for the broken cylinder sleeve. I’ve looked on ebay and other websites, is there somewhere else where I can find a replacement or is there another part that might be compatible?

LA Sleeve. It's what they do.

L.A.SLEEVE

www.lasleeve.com

S F

 

[SemiFast]

I opened the engine and transmission. Everything looks ok, I still have to take some measurement to check if everything is up to specs.

Like Trials said... Something took out that rod. Maybe a hydraulic lock? Probable crankshaft rod journal damage there too.
Have you checked the starting clutch parts yet?

S F

 

[Unkle Krusty]

Wot SF said. A good machine shop can drill out the old and bung in a new sleeve. I had it done on my 53 Bentley. Was handy that a diesel Cat motor had the same bore and stroke. 4 X 4 1/2 or something like that. If I recall your sleeve protrudes below the casting. On my engine they left a 1/8 lip at the bottom so the sleeve could never fall into the crank.
You would love some small diesel engines. Sleeves are designed to be removed and replaced. UK

 

[Jalejandro.velasco]

Update.
I got most of the replacement parts. I haven’t checked the clearance on the rod bearings, but I replaced the bent rod. I bought a set of rods off ebay that claim to be from a same year model bike but have different part number. The ones off my motor have a 426 s21, the replacements have 426 s26. Either way I replaced the bent rod and the new one spins without any force but the other old rods spin much freely, without no resistance. Should I replace all rods? Are the replacemnt rods compatible? Are con rods suppose to spins so freely that I cant balance them standing?

 

[Trials]

Update.
I got most of the replacement parts. I haven’t checked the clearance on the rod bearings, but I replaced the bent rod. I bought a set of rods off ebay that claim to be from a same year model bike but have different part number. The ones off my motor have a 426 s21, the replacements have 426 s26. Either way I replaced the bent rod and the new one spins without any force but the other old rods spin much freely, without no resistance. Should I replace all rods? Are the replacemnt rods compatible? Are con rods suppose to spins so freely that I cant balance them standing?

Here's the thing; the bottom end bearings on that engine are not roller bearings, they operate on oil pressure.
You'll be able to visibly see the damage and wear on them.
The bearing "shells" (they are like a cylinder cut in half) are tolerance fitted to match each connecting rod and the crankshaft journal that it rides on.
You need to understand the technology being applied to even know what parts to order.

Do you have the entire engine apart and the bearing shells out that you can photograph? Don't mix them up they need to go back on the same place with the same rod or not at all. As for the con rod you are replacing, you will need to determine the correct bearing size and tolerance yourself for that one.

 

[Unkle Krusty]

My memory tells me that the rod bearings need to be checked with plastergage. Probably not the right word. There are different colours for the amount of clearance. Not something for a newbie to even think of doing. UK

 

[Jalejandro.velasco]

Here's the thing; the bottom end bearings on that engine are not roller bearings, they operate on oil pressure.
You'll be able to visibly see the damage and wear on them.
The bearing "shells" (they are like a cylinder cut in half) are tolerance fitted to match each connecting rod and the crankshaft journal that it rides on.
You need to understand the technology being applied to even know what parts to order.

Do you have the entire engine apart and the bearing shells out that you can photograph? Don't mix them up they need to go back on the same place with the same rod or not at all. As for the con rod you are replacing, you will need to determine the correct bearing size and tolerance yourself for that one.

These are the top and bottom sides of the crankshaft. I dont know if you can tell but some do have medium ware on them

 

[Jalejandro.velasco]

The thread keeps deleting my replies…
These are the top and bottom crankshaft bearing. Some do have medium ware on them

 

[Trials]

Each of those visibly damaged bearing sets (all of them) need replacing and hopefully your crank is still in good shape,
you really need to refer to the service manual on how to order the right size bearing shells, there is no other way.

and if I was that far into a CB engine I would consider replacing the primary drive chain just because I was already in there.

 

[hogcowboy]

My memory tells me that the rod bearings need to be checked with plastergage.

Really close there UK. It's Plastigauge. Green is about all most need to worry about.

 

[Unkle Krusty]

Really close there UK. It's Plastigauge. Green is about all most need to worry about.

From about 1974. Bruce our mechanic had a CB750 engine apart. I observed. Never did one myself. I would do the 2 stroke cranks. Bruce is also the guy who warned me off CB650 engines. Went on to work for Canadian Honda as their head tech guy. UK

 

[Jalejandro.velasco]

I just wanted to update you guys on the progress. I rebuilt the engine with a new piston and new sleeve from an old motor. I´ve started the motor and even rode the bike. Everything seems better. I have some issues with the carburator but plan to synchronize all four. Also one of the floats appears to have an issue. If I leave the gas valve open, one of the carbs floods with gasoline. So I probably need a new float and needle. But the bike is in better condition I still plan on making some adjustments. Thank you.

 

[hogcowboy]

Thanks for the update. You are probably correct on the carb flooding. Maybe not needing a new float but something is wrong with the needle. Might just need a little extra cleaning but a new one usually works better anyway. Good luck on the synchronizing. The guys here can help if you run into problems. Well, except for @Trials. Looks like we've lost him.

 

[amc49]

MAKE SURE you keep turning the gas off, that is likely what bent the other rod. It leaks into engine and hydraulically locks it to bend rod when you start it. What the vacuum fuel valve originally on bike was supposed to stop.

 

[FlyingTools]

MAKE SURE you keep turning the gas off, that is likely what bent the other rod. It leaks into engine and hydraulically locks it to bend rod when you start it. What the vacuum fuel valve originally on bike was supposed to stop.

Thank you for telling us that vacuum operated petcock was mean to save us from!! I will keep mine on vacuum as I was considering changing on my Virago 920.

 

[amc49]

Can't say on Yamaha bikes but I know it for a fact on Honda, they went through a rash of warranty engine fails because new owners would not turn the gas off. I being a biker in the '60s was trained to ALWAYS do that.