lineboring a flathead

cuzncletus

cuzncletus

Senior Member
While discussing a flathead buildup with a very good machinist/engine man, the issue of lineboring for a center main came up. His opinion was the curved interlocking structure of the mains and block were a significant reason for the flathead's lower end strength, but they would make line boring very difficult. Not only is the tang curved, but it appears slightly crowned. This isn't like lineboring those billions of Brand X's out there. What's the secret?
 
Your right about the difficuly level for sure. I think Ol' Ron uses a special fixture to hold the cap to be turned but he would have to answer that with more detail if so. Taking a little off the cap will allow a minimum for clean up of the bores so special timing gears most likey won't be needed (still enough backlash).

I'm sure a machinist would try to remove as little as possible to true up the bores. If they were way out a better block would be in order.

Kerby
 
You cannot grind the caps, you can surface the cap mating surface, the block... Harder to do, but will work... I've been known to use a flat file on it to close up the clearances-just a bit- on my own stuff... Karl
 
Those who have read my book will have seen pictures of the cap machining method and a discussion about replacing the center main cap. To make a long story short, the new center main cap is first rough bored, then the 3 main housing are rough bored, and finally, the line is finish honed.
 
Very simple operation to set the caps up in a lathe 4 jaw and take enough off to facilitate align boring. They can be done several different ways on a mill also. I have probably made a couple hundred steel caps for flatheads and for me, the lathe is the fastest and easiest way to do the mating surface.
When finishing, I bore to within a couple thousanths and finish hone till my test bar will go in.
DO NOT surface the pan rail for this job.
 
Yes! I intended to comment on machining stock from the pan rail for closing the bores-----It is not the proper approach for several reasons. Align boring, done right, requires considerable skill and attention. The minimum movement of the crank centerline, in relation to the cam, is desirable. Machining material from the bottom of the block to render the housings undersize, rather than from the caps, makes it inevitable that more material must be bored from the block, than the caps, to achieve correct size. This shifts the crank centerline more than necessary.
 
Yes I made a simple fixture that allows the cap to be held in a 4 jaw chuck. I then take 3-4 thousands off the cap, then send it to a shop where it''s aligned bored and honed. This is an expensive process around here. ($350)
 
what am i missing here when we linded bored years ago we would install over sized semi finished inserts.then line bore to journel dia.we had a tool that had a bit that cut the inserts the babit blocks used the same thing.
 
So what would you all recommend on a block that has no caps with it?
Its a very good block (99A) and is only 0.020" over in the bores. I have to fit caps off another engine. would a hone be enough to true the new cap bores to the block? or do i just tell the guy to keep hunting for another (complete) engine.
 
If it were me, I'd keep the block (if the price is reeasonable?) and start looking for main caps for that type engine. You might be very surprised that other caps fit very well, or need just a small bit of attention. If that's a 99 block, it's desireable
JMO, Jim
 
I may have a set of caps for that block, For a reasonable price I could cut them and you could have a local shop align bore it. You can also bore it to 3 5/16 and install a 3" crank. Use 81A heads milled for .050" piston clearance and you have an axle breaker.
 
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