tweaking cast bronze

[bmuss51]

i don't know if there would be a better section to post this but here is my ?
i just finished a new holland hit-miss but the ingniter is not vertical as it should be. so i went to bend it alittle which it is only 3/8 inch square and it broke. so i went to order a new casting and i asked the guy if he had any problems with the arm not lining up. and he came back and told me that they all need alittle tweaking. so i asked how he does it without breaking them. and he came back and told me that he heats the cast arm up till it turns gray which is suppose to anneal the bronze. so i got the new casting and machined it to fit . which i found i had the same miss alignment. so i went to heat it up till it got gray in color and let it cool to room temp. but the same thing happened again.so i went and silver soldered it back together and it looks real good. but when i joined it back together it was off to where it should be, but better than it was as far as alignment goes. so i need to tweak it some more, like about 3/16 inch over a length of approx. 2". but i'm afraid to touch the good joint i have put back together. and if i try to tweak it from that point out, i'm afraid it will snap again.
on the old arm i tried to bend it when it was just starting to get red . but yes it snapped where it was red.so what is a safe way to tweak cast bronze?
by the way the casting at the point it broke looked very coarse and grainy.

HELP, PLEASE!

 

[putputman]

Sorry I don't have an answer on tweaking bronze but I am getting close to finishing a New Holland hit/miss also and am very interested in your problem.

I am not sure what you mean by the igniter having to be vertical. What part of the igniter are you talking about. Could you post a picture of the problem?

 

[cobra428]

Bmuss,
My experience with brass and copper is that they are the opposite of steel when it comes to heat treatment.
Steel when you heat and quench in water or oil hardens. Brass or copper when you heat and quench softens. Letting it cool at room temp will harden it! Don't hold me to this you may want to wait till a heat treat guy jumps in! By the way where did you get the new holland from?
Tony

 

[bmuss51]

the model i'm working is a 1/2 scale of a 1 1/2 hp nh motor that i got the kit from k&j machine in pa. and as for the part it is called a rocker arm. where the exhaust and ingnitor is on the same arm. where to get things ligned up i had to tweak it to the right at the exhaust port and then back to the left for the ingnitor futher out.and trying to get both ligned up is what got me into trouble. because of the casting only having enough surplus to getting it to fit without having to much slop. and i found out that the smaller engine also uses the same head . which i have that kit also. and i will plan for the error in the casting on that motor by shifting the head over to the rocker arm side more. which would have save some grief.
there was some other areas i had to fudge to get things going. like shift holes in the main block like the valve guides where placed to fit the casting instead of putting them to print.

 

[putputman]

Mine is 1/2 scale of a 1/2 HP. I think there is considerable difference between these two models.

 

[abby]

Sounds like the castings are not very good quality , bronzes are not very ductile but should bend a little without breaking. Gun-metal of 85/5/5/5 composition is the best bronze for model castings , it machines beautifully and can be bent by quite a large amount. A good casting will have a fine even grain.

 

[te_gui]

You never wanna try and bend bronze hot. Most are "red short" and will crumble when stressed hot. I am sure ther are exceptions but that is what I have always found. Heat it up and let it air cool then , ever so gently tweak it.

 

[bmuss51]

te_gui: when you said to heat the bronze up, to what color do you heat it to?

i have heated one piece up to cherry red and let air cool and the other piece i tried as it came from the foundry. with the same results. when trying to tweak, it snapped. also when heated up to cherry red and tried to bend, and snap.
i did my tweaking with a crescent wrench closed down to fit the place i was wanting to tweak. and with no warning it went snap. so would i be better of to use a small arbor press with an indicator and go in small amounts?
is there away to tell if the part is going to tweak or just go snap ?
and now that i have an area that was silver soldered back together. would i be better of trying to tweak that area cold? and the reason i ask this instead of reheating the joint to get the tweak i need is because it looks real good and several people can't even tell it was broke at one time. and the second go i probably will not get as good a results.

 

[gilessim]

I'm with Abby on the gunmetal, a few years ago I was making a Stuart H10, and while boring out the big end of the con-rod,I managed to bend it more than 45°! out of frustration for a ruined part, I put it in the vice and it bent back very nearly straight cold!, the shaft had to be turned down a touch but you can hardly tell, otherwise I would have had to wait ages for a new casting ,or spent ages making a new one from solid!

Giles