1934 twin engine rebuild

[george garside]

Barry. a standard 8hp piston is roughly the same as a 7hp + 60 thou and should give you an extra 40cc capacity. However a note of caution the max rebore size recommended was +40 ..but I did run one with +60 many yeaars go. Those John has with skimmed heads may raise the compression ratio usefully - maybe to 5.4 ??.

from what I remember the cam followers should be fine. Bradford CA/CB valves are , I think, slightly longer than 7hp but I vaguely recollect someone sawing a bit off the threaded end to make them fit !

Looking at one of your photos the baffle plates on the crankcase look to be missing ( or have you removed them while dismantling the engine?)

george

[BarryCambs]

Hi George. I'll give John's set a go then. As I have a good pair of spare barrels, I haven't really anything to lose if the 8hp barrels don't work out and as one of my heads is cracked anyway, it will save sorting that and paying for them to be skimmed. He has a good manifold too, so as mine needs work, I can sort that at my leisure and keep it for a spare.

While plenty of bits of the car do seem to be missing, the baffles were there (i assume you mean the semi circular plates either side of the con rods), I just took them off to get the the big end bolts easily. Unfortunately, 2 of the kids and my partner are sick, so no progress this evening. Tony kindly let me have a spare dynamo a while ago and I wanted to check the armature on the original before I decide which one to send off for rewinding. I assumed I would be waiting 2 months for the crank bearings to be re metalled so there was no urgency, but it now seems I could have it back on the road quite quickly.

I had an interesting chat with the guy at our highly regarded local machine shop, as I was assuming I'd need the crank done. I have had "slightly" higher performance engines balanced and the old Moto Guzzi engine will be on their rig once this job is out of the way. Out of interest, I asked him if balancing the Jowett crank would be a complete waste of time, thinking of engine stress, rather than performance. He said he'd do it by adding weight, rather than taking it off if I really want it done, but I'd probably be surprised how good the original factory job was! I think I'll save the money in case I shred the rear tyres with all the new power and have to replace them.

Barry

[Tony Fearn]

Welcome back George!
Barry, I've a feeling that if you are to put 8hp barrels on the 7hp crankcase, there might be a bit of machining to do where the barrel fits into the casing at the flange end. I've never done this, but in the back of my mind it's something you need to ask about.
I also have another niggling feeling about the exhaust manifold studs. Centre to centre might just be slightly larger than the 7hp barrel exhaust studs, so your existing exhaust flanges won't fit without fettling. Again, ask about it.
Tony.

[george garside]

Tony/Barry I might have got the wrong end of the stick! I read it that John Box has 7 hp pots fitted with 8 hp pistons - hence +60 - rather than 8hp pots. If 8hp pots they will need modifying as Tony suggests so best check with John BOx as to exactly what they are.

george

[BarryCambs]

I spoke to John again and they are 7hp pots with 8hp pistons. However, it looks like they are now spoken for. He does have other bit's I'd like though and he thinks he has a CA camshaft and a decent set of timing sprockets. I don't think JCS have pistons, but I rang FW Thorntons who have them in stock +20, 30, 40 and 60 thou, so I'll have a measure up and have my spare barrels bored. Valve guides are available and I'll get the heads skimmed. I don't have the valves out yet, but I will investigate these Bradford ones. They are NOS for 20 quid a pair, so not too much to lose and I'm sure someone would be glad of them if I can't use them.

[BarryCambs]

I managed to buy a complete set of valves today, so was excited about ordering the pistons tomorrow. However, when I tried to measure the bores on the spare barrels I'd bought they seemed to be slightly smaller than the old ones, though they had standard pistons in them. Looking more closely, they are in fact different casting The new ones have large brackets cast on the bottom, I assume for engine mounts and lack the flat front where my mounting brackets fit. What could these be from? I was pondering if I could bore and reuse the pitted barrel if by chance someone only had a odd one, but noticed the skirt on that has been broken too!

I also noticed the head studs are twice the length of mine for some reason. They do look to be in pretty good shape, so hopefully someone can make use of them once I know what they are. All I need to do is find the right ones ..............

[george garside]

the pot with the mounting lugs underneath were used on the first engines with detachable heads c1929 to 1931. It is therefore quite possible that your car has the appropriate holes in the side of the chassis to bolt them on to! Not sure about this but the longer studs could have been because of a different outside shape in the head castings


george

[k. rogers]

My 1933 Saloon has these barrels - I didn't think the rubber mounted type came in until that year?

[Tony Fearn]

My 1933 Saloon has these barrels

Hi Ken, and thanks George.
Ken, do your barrels also have the extended cylinder head studs? We all would like to know; if so, a photo of the cylinder heads would be of great interest.
Cylinder head removal with the engine in situ might be a bit difficult. There's only just enough room with respect to the chassis web to take the normal head off, so it would be impossible to remove that head without messing with the engine.
There are (probably) some people that would know, but don't choose to comment on JowettTalk, even if they are connected.
I just (slightly) wondered if it was anything to do with stationary engines, but don't actually think so.
I have a couple of cylinders in my spares that have the same mountings, but normal length studs.
Tony.
P.S. It just shows that knowledge needs documenting before it disappears forever. Tempus fugit.

[k. rogers]

No, I am quite certain mine has normal length studs as I bought new ones from club spares at the time that I rebuilt it - the heads are the standard ones too.

[PJGD]

I have some 3/8" BSF nuts around me somewhere that came from a twin and which retained the cylinder head, I am pretty sure.
These nuts are of the "extended" type; that is to say there is about 3/4" length of plain tubular shape above which is the regular hexagon. Similar nuts are used to retain the exhaust flanges (and for the same reasons as below).
Nuts like these may well have been paired with the longer studs, particularly if those studs just have the regular thread length at the top.
The problem with the short studs is that they have very little room for stretch when they are torqued up, and thus if anything in the force path were to relax, then the clamp load could easily drop below the minimum threshold.
Clearly, the head gasket is the one item in the force path that is likely to relax, and in the event that it does, then a combustion or coolant leak would result.
The longer studs inherently have more stretch when torqued which will reduce the likelihood of a gasket failure. Look on stretch as being like a spring.
My guess is that at some point, Jowetts were using a head gasket that was more prone to relaxing which obliged them to use the longer studs. Later they were able to source gaskets that had less propensity to relax which negated the need for long studs.
This is of course just a guess, and someone else may know the real reason.

Philip

[BarryCambs]

Dear Philip

Thanks for that comprehensive explanation. Half way through reading it, I suddenly realised that rather than the studs on the spare barrels being too long, it is in fact a case of mine being too short. I do have the extended nuts you describe on mine. I was planing, on Tony's advice, to replace them and JCS do have them in stock. I'd noticed ages ago the studs didn't reach to the end of the nuts which made no sense, but the engine came apart in quite a frenzy due to my "helpers" and I had forgotten about this. I assume they were replaced at some stage. There was an extended nut on one exhaust manifold I remember, but again, the studs are too short. I couldn't get the studs out easily with the engine in the car, so forced some metric nuts on what was left of the studs to keep us going and these are on the list for the rebuild.

As an aside, Tony you mentioned seeing a list of torque settings somewhere. I stumbled across it in the club's Bradford Service spec document. There's actually a lot of good advice in there that is relevant to pre-war engines too. Paul was going to have a look at JCS last night to see what they have in the way of barrels. If it's going to be a problem I might investigate using the mountings on the pare barrels as George suggested. I'm just a bit nervous as everything is so tight. The engine is bolted rigid to the chassis at the moment using various bits of pip and spacers (as is the gearbox), but there is only about 1/4 gap between the heads and chassis rails, so it wouldn't have to move far to start hitting them if it was rubber mounted. I assume they were rubbers originally?

Barry

[Keith Clements]

Do you mean this Bradford doc?

[BarryCambs]

Yes, that's the one.

Barry

[Tony Fearn]

Hello Philip and Barry.
I don't want to unnecessarily extend the saga of the long studs, but I've been thinking about them. I spoke to a knowledgeable 'pre-war friend' (definitely NOT George), and emailed a copy of Barry's photo to him. He replied this afternoon - "yes they do seem longer" -???

Both types of stud, from Barry's photo, have very similar lengths of screwed thread at the end. This seems to suggest to me that each would have used the same type of cylinder head nut. As shown in the sketch below, the thread starts at the cylinder end, away from the hexagon, and then there's a wider bore to allow the stud thread to come through unimpeded. I've posted this sketch before when suggesting why it's better to buy new nuts from JCS if the old one's have been re-used several times. There's nothing worse than giving an old, splayed cylinder head nut that final turn and it goes round and round. (see Philip's post mentioning 'clamp load'.)

I just wonder if the longer studs were used with aluminium cylinder heads:-

Aluminium cylinder head.jpg (18.8 KiB) Viewed 811 times

...or even Bradford heads:-

As you might have seen had not 'Cannot add another attachment, 3 is the maximum' foiled me again, the thickness of the holes in the ordinary pre-war cylinder heads allows for the shorter studs.
I'll post it alone next.
Tony