A10 plunger,cylinder head advice

[Von datoh]

Hi all,the bitsa build is coming along and the engine build is approaching since the chassis is underway.I want to build a “hotter “ motor (still reliable!) so am wondering what cylinder heads fit without to much fuss.I have a later iron head with larger inlet I was gonna use,but am wondering if it’s worth going the ally head route?Later a10 alloy heads are pricey but I’m aware the intake causes problems with the carb fouling the plunger frame.Id like to use the standard oil tank;I’ve also seen people use a7 alloy heads with a bit of alterations.What are people’s thoughts opinions? I have later model large journal crank,thick flange barrels resleeved for standard,plus some 8-1 pistons so far!,any advice appreciated.

[berger]

buy a honda  sorry i went to the pub ,, again , you could spend a fortune on gas flowing a head done by cylinder head shop and dyno balanced crank the list is long but the wallet gets emptied very quickly for a little gain

[Von datoh]

Ha ha indeed that is what my blackbird is for!,I just want to put together a “pepped” up motor that will keep up well in todays traffic.The project is a bitsa so I’m assembling parts as I go.

[berger]

you will easily keep up with the traffic , every motorway is rammed with LETS DIG and speed limits every A and B road is full of LETS DIG and traffic lights even when work is not being done , and i can say for sure you will catch them all up and filter passed them to end up at the front of the line before you know it. 

[Von datoh]

Yes never a truer word said there Mr Berger of our wonderful modern roads in this country!however aside from that no experience/ideas of what fits works well head wise on an a10 plunger ? Thanks in advance

[muskrat]

G'day Vd.
My A7 plunger has an alloy head but your A10 is 1/2" taller so yes the cables will foul the frame.
A std late model iron head will work fine with a little porting to match whatever carb you use (I like the Premier Concentrics).
Tuning is the main ingredient.
Cheers

[limeyrob]

A plunger A10 is somewhere around 36bhp (corrections please!).  The latest Triumph Speed 400 is 40 bhp and is considered to have sufficient performance for 2 up riding in modern traffic.  Everything on the plunger 'Flash is sized to match, tyres, brakes gear ratios, so its best left as it is.  Last week I drove 200 miles in a day in my old Land Rover, 2 1/2 ton and 63 bhp and that was 90% motorway, the rest A roads.  M1 and M25 in the rush hour, but of course that's 25 miles of 50mph roadworks and 40 mph on the M25 just because its the M25.  Top speed on the Land Rover is about 65 but that's painful, realistic is 55mph.  A plunger 'Flash is good for 85-90 (I used to ride one in std trim) and can keep up with modern traffic no problem.
One thing I find on both classic bike forums and Land Rover ones is that new owners assume that modern traffic needs more power, in my experience it does not, what it needs is much better brakes and much better lights and so if you have a "modernizing" budget I would prioritize those over power.

[Von datoh]

I have a Tls front brake fitted in the chassis at present and plan to wire/convert the bike 12 v and have the magneto refurbished,I need it all reliable but as “modern” as can be in the constraints of it being a classic bike.I intend to use it each week and if it proves itself tour up
To Scotland on it.

[Von datoh]

Mr “Muskrat” do you think a twin carb a7 head would fit or for the hassle do you think it not worth the bother and stick with the later iron head I have?

[limeyrob]

Having done quite a bit of mileage on both plunger and swing arm A10s I would not tune a plunger frame bike.  They are lovely bikes but plungers do not handle bumps well.  These are 65 year old grease lubed sliding suspensions.    Plunger is a bolt up frame vs once piece double cradle for the SA.  I've no doubt there's some logic for a sprint bike but on the road?  Not for me.

[Von datoh]

I hear you limeyrob,I’m not looking to go over the top with the engine,just essentially build a “superflash”spec one which had a plunger frame anyway.Ive machined new bushes and made new columns for my rear suspension so am hoping it should be similar to new,what was your experience of the plunger suspension some have said it’s like riding a hardtail till you hit a big bump then it bottoms out with a bang!?

[chaterlea25]

Hi All
Up the gearing, adding a tooth or two to the gearbox on a plunger will give an easier ride, there is plenty in the engine to pull the extra

John

[muskrat]

G'day Vd.
OK you've addressed the front brake issue. Mine is std 7" and sort of slows me down, I did run up the back of a car a few years back breaking 5 ribs and punctured lung 
I did have a twin carb head on the plunger years ago but again yours is 1/2" taller so can't say if it would be OK and doubt if you could use air filters. With an A7 head on an A10 you'd have to chamfer the combustion chamber edge to match the larger A10 bore. Stick with the iron head.
My plunger does wobble around a bit (following riders have a laugh) but I think it handles quite well (and not many pass me). I have made & fitted double dampers up front.
Cheers.
ps: all this talk, I'm going to do the "block" on the old girl now.

[Von datoh]

Maybe sticking with the later iron head is the way to go then,ref the gearing I did consider upping the gearing on the gearbox sprocket although the speedo error would bug me!;for more relaxed cruising

[limeyrob]

Plungers are "different".  They are short travel and there's no dampers, just friction and a rebound spring.  They do absorb some bumps but what I recall as being different is that the rear wheel is not so well located.  This could be wear in the sliders or the bolt up frame.  If you look at the plungers on an Ariel Square 4 they are quite a bit more complex and this is really the only other fast bike with plungers.  Matchless went SA early and Triumph disappeared down the sprung hub hole before realizing their madness.  Plungers are great for chairs, they put a mounting at the rear right where you need it so a lot of A10 plungers survived because they were working hard pulling chairs.