SOHC.co.uk Forums > CB750
Head Bolts
Martin6:
Update: I've posted various questions in other posts, along the way. I'll stick to this thread in future and thought I'd summarise progress.. I'm sorry it's a bit long. Also, please let me know if you see mistakes I'm making 👍.
The engine is out. Head and barrels off.
I found significant damage, beyond repair, to the gasket surfaces between the head and valve cover, where it had been jemmied off at some point. This was definitely one of the leak points. Gouges were deep, across the entire width of the gasket surface, including around the screw hole. A difficult repair, which i would have to put out for welding (if possible). I decided to find a replacement head and cover. I have found a 2nd hand replacement head (with valves) and a cover. They appear to match. Slightly different casting shape on the head's bottom cooling fins, but the gasket face, bolt and stud holes etc. are identical. I've been through the replacement head, thorough clean and compressed air, except for removing the 2 oil metering plugs. With the valve stem oil seals removed, there was a small amount of play in the axhaust valves / guides. I looked at the original damaged head and this was no different. It wasn't burning oil, so I will go with the replacement's guides and valves. Lapped the valves, new stem seals. I think it will be fine. ( after reassembling the valves, springs etc., I realised the springs go in a particular way round, tight coils at the head end. So I've had to do it again!)
I checked the valve timing before I started to remove the cam / tappets. It was out by one tooth of the cam sprocket. When the cam was at TDC, the crank was at 'F' on the points plate. Thankfully, there was no sign of piston to valve contact.
The bolt which is front centre, outside the cylinder head between exhaust 2 and 3, had indeed sheared. It is broken off at a point too deep inside the barrel casting for an eazi-out.
The cylinder barrel: All 4 cylinders were shiny, showing no sign of honing / cross hatching. There is a slight lip in the top of all 4 cylinders. There was carbon on the piston crowns, and the crowns have some wear (a couple have swirl marks, particularly no.2) but no damage. The rings were replaced 2,000 miles earlier. They were free and clean. Looks like the cylinders weren't honed for the new rings. The carbon on the crowns came off easily with brake cleaner and scotchbrite.
The cylinder barrels are now with a local engineering shop recommended to me. They will remove the sheared bolt / check the thread and believe the cylinders will be ok with a gentle hone / deglaze. I will check rig gaps, before reassembly.
I checked the primary chain from inside the sump, the tensioner is within spec at 68.24mm. The cam chain tensioner has plenty left and the chain does not appear worn, it was tight to get off the sprocket (tensioner removed) no tight links and resists 'lateral' movement. I think it's fine. The tensioner rubbers also look ok.
So, I am planning to leave the crankcase undisturbed.
In the cylinder head, I would like to remove the oil metering plugs and clean behind them. Compressed air does blow through ok, but I'd like to check. They don'twanttocome out. Is there a technique to removing them without destroying them?
Committed now! Much to learn. Advice gratefully received. Thanks.
Martin6:
The cylinder barrels are back. Light hone and broken bolt removed. I've measured all the dowel clearances and they look fine. I'll be using the David Silver supplied gasket set. I will check the o-rings match the gasket thickness. Which o-rings is it that are potentially too thin?
Once I've cleaned the bores and oiled them, I think I'm ready to reassemble. I'll follow the book. Haynes and factory, although both seem to focus on earlier top ends. I'll aim to put the cylinders and head on today and torque it down. Mine had loose nuts and bolts when I took it apart, so I'll leave it overnight and check the torque again, before moving onto the cam and valve gear assembly. This is a first and I'm nervous ::)
- Are there any tips you would give?
- I've seen mention of applying a light coat of gasket sealant to the plastic / rubber pucks. Is that a good thing to do?
- gasket sealant anywhere else? It seems odd not applying anything to the gasket surfaces at the base of the barrels and the head.
Thanks, Martin
Martin6:
Hi, My gasket kit does not include four O-rings. Those that go around the studs either side of the rear of the cam chain tunnel. I think it is 2 between the crankcase and cylinder block, then again 2 between the block and the head. My old ones are distorted, but are about 16mm ext dia, to give a rough idea.
Does anyone have the right dimensions for the rings and a good source?
Thanks, Martin
Pauarc:
Hi Martin
Part fish for head
Paul
Martin6:
--- Quote from: Pauarc on October 29, 2023, 05:07:23 PM ---Hi Martin
Part fish for head
Paul
--- End quote ---
Well done, thank you. 11mm x 2.5, = 16mm o.d. Must be it.
Appreciated.
Martin
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version