do I need to replace my main bearings

[Sp1john]

hello everyone.. I am john from Nottingham in the UK..  I have had a honda cb750 f1 (28000 miles) now for 15 years with the intention of rebuilding the bike.. av finally got round to do it 🙄.. the engine has done 28000 miles and is in now in bits.. I have plastigauged the main bearings.. this is reading 0.05 to 0.63mm.. can anyone give me some advice on weather this is ok.. also I am in need of a set of std rings any advice as to we're I can get them from .. the piston has 392 in the casting next to the pin.. thanks john

[Johnny4428]

Hi John welcome to the forum! There is very helpful and knowledgable people on here as well as very experienced that will advise. That’s why I’m not going to offer my help 🤣

[McCabe-Thiele (Ted)]

A good starting point is to download the manual here (unless you have done that) then see how the crank journal tollerances look in the tables. Check you bearing codes if the colours are visible.

[Oddjob]

It depends on condition of the bearing surfaces as well John, when you have 5 posts then put up a pic of them, mark each one before removal, they MUST go back in the same place they came from.

[Sp1john]

Hi John welcome to the forum! There is very helpful and knowledgable people on here as well as very experienced that will advise. That’s why I’m not going to offer my help 🤣

😂👍thanks for the welcome.. forum's like this one are invaluable.. 👏

[Sp1john]

A good starting point is to download the manual here (unless you have done that) then see how the crank journal tollerances look in the tables. Check you bearing codes if the colours are visible.

hi Ted
thanks for your advice.. I have checked and according to honda they are top limit to slightly oversize. I have a clearance of 0.05 to 0.065mm. but I have heard that some people run them with more oil clearance ..

[Sp1john]

It depends on condition of the bearing surfaces as well John, when you have 5 posts then put up a pic of them, mark each one before removal, they MUST go back in the same place they came from.

thanks for your advice... I did try and post photos with this topic but it wouldn't allow me to.. I know why now
.. thanks👍

[Sp1john]

here is the 2 pictures of my dilemma

[Oddjob]

Need actual pictures of the face of the shell bearings. They can corrode due to acid in the engine oil over the years, so even though the clearance may be ok the shell could be ruined.

[McCabe-Thiele (Ted)]

Now that you have split the crank cases imho it would be essential to replace all of the crank shells - mains & big ends.

Not sure if you did a compression test before dismantling, I would also rebore at the same time so A1 crank shells is a must.
Although my experience with petrol engines is mostly car based I would never just fit new rings.

Just my thoughts - now you have the engine in bits replace anything that you can afford. Chains primary drive bearings etc.

[Sp1john]

Need actual pictures of the face of the shell bearings. They can corrode due to acid in the engine oil over the years, so even though the clearance may be ok the shell could be ruined.

thanks.. al put some on today👍

[Sp1john]

here are the photos of the main bearings..sorry for the quality of 2.. should I still use them 🤔

[McCabe-Thiele (Ted)]

I gotta say those mains look amazing - you must have done the oil & filer changes regularly.

[Sesman]

I’d say if it’s the first tear down and the crank is in good shape it all depends on your mission and riding intentions. If your are doing a full restoration I’d replace them.If you are happy with the engine and don’t intend to rack up the miles and are operating a strict budget, you might want to use the cash saved elsewhere. I can’t help myself and replaced all shells, main load bearings, dampers and chains….it’s just the way I’m made.

[Laverdaroo]

Same here,  mine being an anchor to start with pretty much swayed the decision mind but I'm in Phil's camp too.