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valve seats

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Pete Berry22/11/2009 12:58:18
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24 forum posts
7 photos
I am experimenting with building 4 stroke petrol engine(my first venture into this field) the engine has an aluminium cylinder head and s.steel valves 8mm dia, the seats are cut directly into the aluminium head ie no speacial seat. question 1. Is practice ok . Question 2. how wide should the seating be. any other info would be appreciated.
Pete berry
MichaelR22/11/2009 19:39:38
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338 forum posts
58 photos
Hi Pete.
I think you would be better making bronze valve pockets and press fit in cylinder head, you will get a better and long lasting seat for the valve.
 
mgj22/11/2009 20:02:30
1008 forum posts
14 photos
Well the conventional way is to use bronze guides, shrunk or pressed in, and iron seats - again shrunk in.
 
That engine has very long pockets for its inlet tract, which is unusual since it lengthens the heat path in the valves.
 
I imagine hes going to get round to
 
1. Sharpening his milling cutter.
2 Levelling his milling head
 
at some stage.  Because he is out by a mile with those dogs teeth.
Pete Berry22/11/2009 22:36:29
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24 forum posts
7 photos
Hi stick
I will have a go at installing bronze valve pocketsif there is room to do so. The engine is complete, built to my own design and form drawings of other engines. It is a twin cylinder over head cam 1 inch bore x 1 inch stroke. It sort of trys to start when cranking over with a drill, but at present some of the valves leak .Thanks for the info.
Regards
Pete 
Pete Berry22/11/2009 22:40:50
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24 forum posts
7 photos
Hi Meyrick
looks like I will have to install valve seats or pockets.
thanks for theinfo
Regards
Pete
chris stephens22/11/2009 23:21:26
1045 forum posts
1 photos
Just to add to Meyrick, cast iron is/was used for valve guides as well as seats, at least on some older vehicle's engines.
Perhaps the "milled" surface is deliberate, so it can be used as a key for his gasket goo?
One must be charitable. you know 
 
chriStephens
mgj22/11/2009 23:33:50
1008 forum posts
14 photos
Are the valves leaking, or is the timing too advanced and its spitting back through the inlets?
 
Depends on whether the valve timing is extreme and you have a lot of valve overlap..If the valve seats have all been lapped, I'd be inclined to retard the ignition a bit more before stripping everything down.
 
Take the valve cover off and check that the ignition timing is not triggering before the valves are fully closed. You are firing on the power stroke? sure? Maybe its wasted spark so it doesn't matter.
Pete Berry23/11/2009 17:29:48
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24 forum posts
7 photos
Hi Meyrick
The valves are leaking, I puta water guage onto the inlet of the carb and turned over by hand with the tappets looseie cam not operating the valves. Valve timing is ok but not sure if the distributor is producing an additional sparkat the wrong time. When building the head I was refering to the Nemit 15 drawing for ideas which seamed to show the valves seating directly onto the alloy head. My engine a twin shares one carb.and when looking at the plugs after trying to start it one of the plugs is a bit sooty.
Regards
Pete
mgj23/11/2009 18:22:05
1008 forum posts
14 photos
Pete, sorry but it was only a thought.
 
Lap the valves in with lapping paste. I would have thought that .030 - .050 of contact would have been plenty plenty.. On a full sized racing engine we only used .075 and I'll bet the compression ratio on that and BMEP was much higher than on yours. (Triple springs to cope with bounce and high flank accelerations on the cam!)
 
I wouldn't worry about one plug being sooty (other than it will affect starting). You need to start on the rich side, and since inlet tract velocities will be low, you will not be getting good atomisation. If it persists at higher speeds, thats a different matter  and you will need to look at jet size, jet position in the throat, and emulsion tube settings if you are using an emulsion tube. Nor at low speeds can you draw much of a conclusion as whether one cylinger is robbing the other. You could just be getting better sealing in one cylinder.
 
The other thing that might be worth checking is the valve spring seat. If that is not truly level and set at 90 deg to the valve guide, it tends to cock the sping over, which pulls the valve out of true. (Or can do so)
 
Your head gasket is not leaking by, and the head is actually flat? Tat might be the source of the leak. I'd use soapy water to tell you exactly where the leaks are.
Pete Berry24/11/2009 10:25:42
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24 forum posts
7 photos
Hi Meyrick
Thanks for the info, I will if possible  (other half permitting) check the item you sugested next week, very usefull info. I will let you know the results.
Regards
Pete.
 
Bowber26/12/2009 19:59:29
169 forum posts
24 photos
The Edwards 5 radial engine runs the valves straight in the Ali head, I think some people do the brass/bronze insert thing but most build as the plans, can't remember the seat width though.
 
A small Italian company used to make 250s for racing in the world road racing champs and the isle of man, they ran the valve direct into the head as well, said they ran it first to work harden the seat then raced them, very good heat transfer apparently.
 
Most modern racing motors will use about 1mm seat width, my Jawa speedway motor has single springs as well, progressivily wound and conical with tiny stems on the valves.
 
Steve

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