Topic: 350F crank bearings

[jet72]

I'm new to the forum and the Honda 4 apart from owning a 71 CB 750 years ago when they were just used bikes and my latest project.
I have owned more bikes than I can remember and this is my first four stroke rebuild.
I recently bought a 72 350F with a seized cam chain tensioner and the associated issues that go with that. The main bearings are passable and I have a question regarding bearing sizing.
The mains are marked standard and have D2F-A markings. There is no colour mark on the edge of the bearings. Reading the workshop manual, "A" would indicate "black" bearings.
The case stamps read BBBBC.
I'm going to plastigauge the mains to get an idea of the clearances. The engine is low mileage and very clean inside.
If the clearances are ok, would I be safe ordering a set of black bearings?
I'm a retired automotive tech and on a pension so can't afford to make mistakes.
Any help or wisdom would be appreciated and am I on the right track. Thanks to all.

[HondaMan]

I'm new to the forum and the Honda 4 apart from owning a 71 CB 750 years ago when they were just used bikes and my latest project.
I have owned more bikes than I can remember and this is my first four stroke rebuild.
I recently bought a 72 350F with a seized cam chain tensioner and the associated issues that go with that. The main bearings are passable and I have a question regarding bearing sizing.
The mains are marked standard and have D2F-A markings. There is no colour mark on the edge of the bearings. Reading the workshop manual, "A" would indicate "black" bearings.
The case stamps read BBBBC.
I'm going to plastigauge the mains to get an idea of the clearances. The engine is low mileage and very clean inside.
If the clearances are ok, would I be safe ordering a set of black bearings?
I'm a retired automotive tech and on a pension so can't afford to make mistakes.
Any help or wisdom would be appreciated and am I on the right track. Thanks to all.

Chances are, the bearings don't need replacement. Those are replaced far too often in these rebuilds. Clearance of about 0.0018"-0.0020" is about nominal for that one, too. The Black would provide 0.0008" to the "B" journals, so you would have to do a whole new break-in sequence with it.

Of greater importance are the bores. Chances are VERY high they need to be bored round again, because they are not round now. If you just try to re-ring it, the rings will never seat. This is common across all of the SOHC4 and DOHC4 engines before the middle 1990s, for reasons I can explain if you want to know the details. Boring it round now will make it stay round, essentially forever. Also: when you have the cylinders off, make sure to deck the top of those cylinders at least 0.010" (0.020" is better) or it WILL leak oil when you bolt it all back together and ride off. I can explain why that, too, if you care.

Check with Cruisinimage for a set of pistons. I just got a set for my 350F last Fall, only about $115 at my door. Use the 0.5mm oversize set for best results. A big-bore kit will only restart the warping situation, and there is no recovery above 393cc. It will deliver more power to bore to round again, and it will stay there. Right now it has about 70% of the original power.

[Trevor from Warragul]

If you do go for new pistons, be careful who you choose to rebore the cylinders. It is ridiculously easy to overbore a 350F, as the clearances are very, very small.

[bryanj]

Numbers on the shells mean absolutely nothing you work from letter on the crank, casing and rods plus the table in the manual.
As long as you are putting the shells back exactly where they came from you will probably be OK if you mixed them up you are stuffed

[jet72]

Really appreciate the response. The ring issue was my next question. I was hoping to not have to do a re-bore but there it is.
My local shop here in Calgary does top notch machine work but boring is $130 Can. a hole.
Time for a think session. Thanks again.

[HondaMan]

Really appreciate the response. The ring issue was my next question. I was hoping to not have to do a re-bore but there it is.
My local shop here in Calgary does top notch machine work but boring is $130 Can. a hole.
Time for a think session. Thanks again.

Per HOLE!? Wow! That's almost how much my local machine shop charges for all 4 holes. The comment above about clearances is particularly important in this engine: it is 0.0004"-0.0008" allowed clearance (that's not a typo!). I even did my own 750 to just 0.0006", and it runs SO well, much cooler.

[bryanj]

I was quoted £250 plus 20% tax for a 500 four block in uk and as i had several to do, including a 605 big liner conversion, i bought a used boring bar that i can sell later

[jet72]

I'm thinking of parting the bike out or selling it on as a project. Budget restraints preclude having it re-bored and buying a piston, ring set. There is also our weak Canadian dollar which only compounds the matter. Add 30% to purchase price and shipping costs.
It has a 2 into 2 header of unknown origin so the plan was getting a Delkevic system.
Regarding parts. I found only one fuel tank on e-bay which was damaged and rusty inside for around $350 US. My tank still wears it's original Olive green paint with normal age related wear and is spotless inside.
There was also a grab rail listed for over $300 US!! Mine is like new. Makes a good case for parting it out. Any thoughts or comments out there and sorry for the rant.

[Maraakate]

I'm new to the forum and the Honda 4 apart from owning a 71 CB 750 years ago when they were just used bikes and my latest project.
I have owned more bikes than I can remember and this is my first four stroke rebuild.
I recently bought a 72 350F with a seized cam chain tensioner and the associated issues that go with that. The main bearings are passable and I have a question regarding bearing sizing.
The mains are marked standard and have D2F-A markings. There is no colour mark on the edge of the bearings. Reading the workshop manual, "A" would indicate "black" bearings.
The case stamps read BBBBC.
I'm going to plastigauge the mains to get an idea of the clearances. The engine is low mileage and very clean inside.
If the clearances are ok, would I be safe ordering a set of black bearings?
I'm a retired automotive tech and on a pension so can't afford to make mistakes.
Any help or wisdom would be appreciated and am I on the right track. Thanks to all.

Chances are, the bearings don't need replacement. Those are replaced far too often in these rebuilds. Clearance of about 0.0018"-0.0020" is about nominal for that one, too. The Black would provide 0.0008" to the "B" journals, so you would have to do a whole new break-in sequence with it.

Of greater importance are the bores. Chances are VERY high they need to be bored round again, because they are not round now. If you just try to re-ring it, the rings will never seat. This is common across all of the SOHC4 and DOHC4 engines before the middle 1990s, for reasons I can explain if you want to know the details. Boring it round now will make it stay round, essentially forever. Also: when you have the cylinders off, make sure to deck the top of those cylinders at least 0.010" (0.020" is better) or it WILL leak oil when you bolt it all back together and ride off. I can explain why that, too, if you care.

Check with Cruisinimage for a set of pistons. I just got a set for my 350F last Fall, only about $115 at my door. Use the 0.5mm oversize set for best results. A big-bore kit will only restart the warping situation, and there is no recovery above 393cc. It will deliver more power to bore to round again, and it will stay there. Right now it has about 70% of the original power.

I'm interested in knowing about the reboring/rings issues pre mid-90s hondas.  Advancements in metallurgy?