Honda-SOHC

Thanks guys for your responses, got the block/pistons on ebay last night, so have a back up if mine are completely knackered.

Thanks for the advice BrianJ

Hi all, just registered having come ax you while hunting for info on tearing down the engine on my poor old seized 400 four.

Im afraid it is a bit of a sad story, the bike was my and my ex's mode of transport back in the 80s but, long story short got stored away in my mum's garage back in the 90s. When I came to retrieve it from the garage in 2020 when Mum died it turned out the roof had caved in and it had been exposed to the elements for a long time by the state of it. So I van'd it up to Cambridge with a view to renovating it when time permitted. Having just attended my ex's funeral, I thought in remembrance, now would be an appropriate time to have a go at getting it going again.

So although it only has 25K on the clock, the state of the bike is very poor (see attached photo hopefully), anything that looked like chrome is now very rusty, but a recent visit to David Silver (museum originally but then the parts dept) suggests a lot of the bits can be replaced if not restorable.

However the main problem is the engine is seized and no amount of applying pressure to the points nut will free it. So I intended to remove the plugs and fill the pots with penetrating fluid to see if I could free it without having to resort to tearing down the top end, but the plugs were all seized and over a period of several weeks I have been gently applying penetrating fluid to them and weedling them with the result 3 have finally come out ok, but the 2nd one in from the right has snapped off leaving the thread in the head, so now I have to get the head off anyway to get that taken out.

Over the past weeks I have also been working on engine mounting bolts, engine cover screws etc and all are now free with a couple of exceptions, the main front mount bolt which is completely seized even after a lot of soaking and working, this is problem No. 1, so will need to borrow a friends torch and get some heat on that, and also the oil filter bolt which is rounded off, problem no. 2. The tensioner lock nut is free but I daren't apply any serious pressure to the tensioner bolt itself at the moment. I have also removed all the externals like crash bars, coils/plug leads, exhaust, carbs etc and given the engine a bit of a clean/degrease as it was filthy.

So finally this weekend I got going and removed the breather plate and cam/rocker housing and it all looks good inside, journals all good. I also got the 2 tensioners out ok but the chain is still too tight to go over the top of the sprocket.

Because the crank is currently unmoveable, I was hoping that the cam sprocket would be in a position for me to get at the 2 screws securing it to the cam, one is and is now loosened but the other is just down too far to get a spanner on it. So that is problem no. 3. I dont want to split the cam chain if at all possible. I have come ax a post of someone grinding down a 10mm spanner until he could just get it on the bolt, so that may be the only other option.

Problem No.4 is the 2 head nuts in the 2 inner plug wells are extremely rusty and Im anticipating will be well seized and I dont want to damage the studs, so not sure how to proceed there.

I have also noticed that although when in neutral the driven shaft is free, when I put in in gear even with the clutch pulled right in it is not, so am assuming the clutch plates are seized too, but that is down the problem order at the moment.

So any advice on any of these problems would be greatly appreciated.