Hello all,
I have all the parts now to rebuild my 650 engine and have been cleaning parts up. While cleaning the head I noticed that there is a crack between the two valves in both combustion chambers. Obviously I would prefer to not buy a new head, but I don't want issues down the road either.
I will be rebuilding this head with all new parts and increasing the intake valve size to 34mm.
I have attached pictures of the two cracks. What do you think? Don't worry about it? Have the shop doing the head work attempt to weld it? Use an aluminum brazing technique myself?
A new head is around 200 euros.
I have another 650 engine that I will remove the head from and see if it is in better condition, but wanted some advice in general.

John

This is a very common issue
An aluminuim weld repair would be stronger than before and cost beans
A specialist job in my opinion by the machine shop doing the head work

If you have it welded then they should remove the inserts first.
To be honest you could leave well alone. They crack there due to the heat build up over a very small surface area but generally it is not deep enough to cause a problem. The head is fitted with 2 steel inserts of which the valves seat on. It is for this reason that there is minimal aluminium between the seats.
If it is only superficial leave it. If you weld it chances are it will crack again.

John, have just had my modified panda 30 head repaired with welding, the shop welded the cracks and then recut the valve seats, but I left the standard valves in to maintain the maximum of material between the valves ( i don't need the change the valves anyway) Andrew is correct the head will be stronger.

Ian.

My feeling is always---"if you can feel them with your nail, repair them". The old valve seats have to be removed (to fit the seats for the bigger valves), so get the machine shop to repair the cracks at the same time. I would also be tempted to get the machine shop to give the head a very light skim to ensure absolute flatness (but DO NOT remove the exhaust-relieving ring).

Hi Tom,
Definitely can feel those cracks! If my other head is not in better condition then I will have this head welded. The intake is going up to 34mm but I am staying with the standard 28mm exhaust valve. I have no plans to increase compression ratio by milling the head. My compression ratio is already 8.0/1. With a 0.5mm head gasket that should give me just enough of a compression bump for a few more horsepower.
Thanks to all for your advice.
John

Are bigger valve seats available or the machine Shop will fabricate them? When you do the work can you please post a before and after picture?

Both my panda heads have the same cracks, the one I have in my Gamine has been there for the last eight years with no problems. The valve seats are 10 mm deep and the crack would have to reach this depth to reach the inlet or exhaust chambers in the head to compromise engine function. Well this is what I was told from and old Italian 500 race engine builder.

Ralph

Very interesting Ralph, and I see the logic in his thought process. John, my suggestion to have the head skimmed is only to ensure that it is absolutely flat---very little need be taken off, and just ensuring flatness will not I suspect take off enough to materially increase the compression ratio.

I have run heads with small surface cracks between the valves with no problems. I have yet to see a head where the cracks go much more than skin deep. The question I would ask is why do they crack? Is it a design fault that effects the larger engines ? Could it be due to natural stress relief as the casting settles in use ? Maybe it only happens when the engine has run too hot. I guess that we will never know but if it was a progressive problem then I am sure that someone will have posted pictures that go beyond a surface crack. I my mind I think that there may be a danger in welding as the localised heat may set up new internal stresses in the head.

I have been looking at the various threads that exist regarding valve enlargement and to help me obtain the ideal Bore/Stroke Valve ratio I've fallen on the following web site -
Sorry URL won't transfer google calculating intake and exhaust valve sizes.
Basically my Panda 30 Heads come with valves of In 34mm and Ex 28mm as standard, using the calculator I can adjust the required max rpm to find ideal sizes to suit i.e. using the Panda 30 HP rpm of 5500 I get In 33.04mm and Ex 28.08mm, but using 6000rpm the sizes rise to In 34.51mm and Ex 29.33mm
The Panda engine was rated unbalanced so with proper preparation you can go to In 37.27mm and Ex 31.68mm at 7000rpm. Although the question of getting the valves into the available space can be offset by special cut seats and valve angles the room available is finite. There are racing engines available at hellish prices that will apparently run to 8000rpm but you need deep pockets!!! I do wonder how fragile the valve seats are with the largest possible valves and it must take some clever (expensive) engineering to stop everything colliding inside

Ian.

I checked the head on my other 650 engine and it is worse than the head on the engine I am rebuilding! See attached pictures.
I will just get the first head welded up by the shop that does the head work.
John