I am currently doing a 6.9 build to go in my 93 F350 in place of the 7.3. The block I am using has been bored .030 over already. Unfortunately the pistons are cracked in the combustion area. Otherwise everything looks about perfect.

I just finished using an inside mic on the cylinder bores and and came up with these results.

Cylinder;
1 4.031 - 4.032. Out of round .001
2 4.031 - 4.032. Out of round .001
3 - 8 4.031. Out of round less than 0.001

I checked each bore at the top, center and bottom on 3 different axis. I was really surprised they were so consistent.

My questions are about replacement pistons and boring the cylinders. Based on your experiences do you think I should get new .030 over pistons and just hone the cylinders or should I do a minimum bore and get pistons in that size?

What is an acceptable jump up in bore? Realistically what could be machined accurately. Could I jump from .030.to .040 or would .010 be too little to machine accurately?

Thanks.

 

I've seen many a motor put together with worse bores than that.
Boring is a thing of the past, Unless the bore is off center, Honing to size is the preferred method today. And being the next size is .040 over, .010 is more than enough room for cleaning up those holes.

-Enjoy
fh : )_~

 

Thanks for the feedback. I will probably go ahead and move up to the next size piston. Is there a specific lower compression piston available for this motor? I have read about machining the tops of the pistons but I might want to go down below 20:1. I don't trust that one can be machined that much safely. I am aware that starting could become an issue but at this time it is just an idea I am toying with. It will for sure get lowered compression. How low is what I am trying to determine.

 

Yes, there should be a lower compression piston available. These engines have very tight running clearences between the valves and pistons that make block or head resurfacing unwise. Piston makers decided to offer a shorter piston for this exact situation. I think they are 10 thou shorter than stock.

If you are having a machine shop do any work on the engine, they will probably be able to help you with that. Otherwise you can have the stock pistons cut down slightly. I had new stock height pistons cut down by 20 thou. Another build here was 40 thou off and I know of another 6.9 that had 60 thou taken off them. Any good machinist with a lathe can cut the pistons down for you however much you want.

In the case of the engines with 40 and 60 thou taken off them, they are not exactly being babied, so 20:1 compression does seem realistic if you want to go that far.

 

I would sure hate to take off .010 with a hone.

 

if i had to build one i would go all out and go .60 or .80 off the tops, but im crazy i guess, i just want alot of power lol

 

Been there done that many times, With a Sunnen hand hone on a 1/2" drill.
Take your time, keep it cool and keep measuring.

The new honing machine centers can cut huge amounts in short time. If I remember correctly either Sunnen or Haas has one that will cut something like .038 per stroke! That is some serious cutting with a hone.

The big thing about Honing is the fact that it will follow and existing bore and keep it's centerline, With a boring machine the possibility of boring off center is high.

A con of honing is the heat generated expands the bore.

-Enjoy
fh : )_~

 

Got the block stripped down today and ready for the machine shop. It'll go next week. my machinist most likely has a boring bar not a hone. I am not worried however as he has done work for me before and is very **** about quality. He does alot of farm tractor motors and race motors locally. He works on alot of diesels. I also have someone to machine the pistons. Now I just need to get things rolling.

Thanks for the info.

 

how much are you going to take off the tops of the pistons?

 

I had determined that if the pistons that came out of the motor were good I would take .050 off of them. I still want to do that.

I was considering leaving the outer .375 of the piston diameter alone and just removing material from the center. I know on gas engines it is important to maintain your quench area. I am not sure if it comes into play on an IDI. My feeling is that it won't matter but there is no science to back that up.

 

What compression ratio will .050 give you.

 

about 20:1, maybe even a little less

 

Are you planning on running high boost like dave s. is?

 

50 off will be about 19.68 to 1.

Low RPM torque will drop some till you start getting boost and cold starts when the temp is low will be harder.

With 40 thou off mine, my coldest start without being plugged in was -10, and I don't think I would want to try much colder if it has to be dependable for starts.

My machinist gave me perfect cylinders with only .002 to play with when I did mine.

 

Yes the plan for this motor is high boost. I have ported the heads already and am planning on a twin turbo setup in due time. My reasoning for the twins is less piping to the turbine to lose heat in. Due to a lack of time available right now (Been working 7 days a week the past month) I will originally be fitting the factory turbo back up until I can fabricate the manifolds.

I have been sitting on a Moose pump for over a year now and my goal is to try and put enough boost into this motor to make black smoke impossible. Probably won't get that much air in there but I am going to try.

On the subject of high power IDI's, does ARP make a stud kit for the main bearing caps?