Honda CB750 Oil Pump and Primary Chain & Bottom End Check | Cafe Racer Project Bike | No 11
In this episode I remove the sump to check the oil pump and the primary chain from my CB750 Cafe Racer project bike.
I have already had some surprises with the barrels and pistons for this engine, and I have found some debris in the top end that gave me cause for concern. I don’t really want to get into a full engine rebuild for this bike, but I do want to be reasonably sure that it doesn’t have any serious problems.
I decided to take a look in the sump to see if there was anything that would cause concern, and I also wanted to check the primary chain as well while I am in there. If possible I wanted to check inside the oil galleries for anything lurking there.
Vernier caliper: ebay.us/uyBP1D
Basically the oil galleries are where some of the oil passages in the bottom half of the crankcase have been machined, then capped off with round plates that thread in and have a hex moulded into the top of them. The parts manual describes them as “Cap, Oil Path” and they also have an o ring sealing them. There isn’t any real reason to remove them other than to take a look. I was hoping to see if there was any swarf or debris in there, or more accurately I was hoping to see that there wasn’t :)
Sometimes they will come straight out but as on this engine they have been powder coated over in the past I thought this was unlikely. Added to this the hex moulded onto them isn’t really up to the job of removing them after 40 years. Unfortunately I rounded the heads of them straight away, so I decided that rather than cause more work I would leave them. It turned out that with the sump removed you have reasonable access behind them anyway, at least enough to check that they are clear and that there wasn’t a buildup of debris.
All of the sump fasteners have been replaced by stainless Allen head screws by somebody in the past, so they came out really easy. There was some “fluff” with a few metal shavings on the oil pump strainer, but not a lot. The pan of the sump also had some black sludge in it, but checking it with a magnet there didn’t appear to be any metal in it.
The metal shavings could well have been from the helicoils that I found in the head so I am not really concerned about them. They certainly were not parts of broken bearings or other major components.
I decided to open up the oil pump to check it inside. I found that the sides of the rotors did have light scratches where something had been in there in the past. I do not have a lot of experience of looking inside this kind of oil pump so I needed to do some research.
Here's the awesome modular engine stand I use: ebay.us/ruEvbw
One of the problems with internet research is it is hard to judge how authoritative the writers are. A lot of information is just amplified and passed on without the authors having real experience of what they are writing about.
There seemed to be a lot of opinion that if there are scratches inside your oil pump then it is scrap. But I also found some that said there is nearly always some sort of marking on the outside of the rotors in these pumps, and that if the clearances given by the manufacturer are right then the pump should be fine.
I know that the engine had good oil pressure from the short time that I ran it, so I know the pump was working. The oil goes from the pump to the filter next ( I know that some feeds off to the gear cluster, but that is less of a concern ) so anything going through the pump would be filtered out before it got to the main engine bearings. I checked the filter when I originally removed this engine, and there was nothing that I could see when I checked it.
On balance I decided that the oil pump should be OK and that I will run the engine with it. A bit of common sense says to give it an early oil change, and probably check the strainer on the pump again when I do.
I also checked the primary chain tensioner. The measurement of the deflection of the tensioner when new should be around 65mm, and Honda say when it reaches 70mm the chains should be replaced. The measurement for this engine was 66.8mm so well inside the Honda spec, so not a concern.
Overall I have decided to run the bottom end as it is, checking it during the run in period for the top end. I don’t think that this is too stupid a decision. I know a lot of the keyboard warriors will predict complete mechanical failure within minutes, but I suspect that it will be just fine. Anyway it will be interesting to find out.
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