TheSamba.com :: View topic - Flycut head effect on push rod tube length

Wed Sep 17, 2008 10:53 am

Heynow,

I'm about to remove the heads on my T1 upright engine to replace the PRT seals, and I vaguely remember having them flycut 15 years (and 60,000 miles) ago when I put them on. Will this affect the 190-191 mm tube length that Bentley recommends, and if so, would it be safe to subtract from that length the amount by which the heads were cut? If that's the case, then is it easy to determine how much was cut?

I can post a photo when I get them off, if it helps at all..

Wed Sep 17, 2008 10:56 am

It shouldn't because there should be shims under the barrels to compensate for the removed head material, otherwise your compression would be crazy high.

Wed Sep 17, 2008 12:36 pm

Odd question, you have 60K miles on them now, did not mention having concerns about tube length when you first installed the heads 15 years ago, so why now?

If you really want to know, cc your heads, the dish in the piston if there is any and measure the deck height, from there calculate your compression, compensate with barrel shims to adjust the compression, you may need shims to get the right rocker alignment.

Now, isn't the 190-191mm a starting point to stretch the tubes prior to install? That would be 7.50" +/-, I doubt a .010 or .015 cut will affect the crush on install.

If you have gotten 60K miles and it ran good as it is I would just lap the cylinders to the heads for a good seal, put new PR tube seals on and put it back togther.

Wed Sep 17, 2008 1:51 pm

15 years ago, I didn't know much- just followed Muir in rebuilding.

In fact, can't say I know much more *now* about it, but then I happened across the language regarding flycut heads/tube length on this page and thought maybe that had something to do with my leak-prone PRT seals. Over-squashed or something...

Here's a cleaned up head for review: I don't have a stock head for comparison, but I suspect these weren't cut that much so I'll probably just put 'em back on and be on my way.

By the way, how does one "lap" the cylinders to the heads?

Thanks for your input.

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Wed Sep 17, 2008 2:29 pm

Is that a crack in the left cylinder between the intake and exhaust valve?

Lapping, each cylinder is lapped to the corresponding head chamber. You can use comet or valve grinding compound, you put some around the rim of the cylinder, then rotate the cylinder say 10 degree swing back and forth. I use a black marker to mark the cylinder rim and when all the black is gone the cylinder and head are for a better word "joined". Clean off the lapping compound and do the other three, make sure you keep them marked as to where they go.

Wed Sep 17, 2008 2:46 pm

bussman,
I too will soon be lapping my cylinders. I was told to start with the cylinders on a sheet of glass to get them flat, then lap the cylinders into the head. Would you skip the first step? Also, you rotate just 10* to either side of where the cylinder seats? Any harm in spinning more?
Thanks,
John

Wed Sep 17, 2008 2:48 pm

Quote: Is that a crack in the left cylinder between the intake and exhaust valve?

Yea, however, I'm in denial about it so please don't mention it...

Actually, since you spotted it, how long might my denial last before it turns into the reality of replacing it?

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Wed Sep 17, 2008 3:55 pm

Denial is over, reality is here. Yep that head needs fixed, in fact both heads should be redone at the same time. That or get a set of new heads. You will still have to lap them.

Phip, your are right, first make sure the rim of the cylinder is smooth/flat. I keep the rotation to a minimum, always have, probably doesn't make much difference, makes me warm and fuzzy to know that the orientation of the two has been kept close to one area.

Wed Sep 17, 2008 5:49 pm

the cylinder rim will be flat- it was machined.

but there can be burrs from dents on the outside or inside corners. when using glass, i would sand only enough to knock down burrs.

i would use only a machined plate to sand out scratches and dents. i only do that in the case of a scratch that completely traverses the mating surface. and then, i sand both cylinders equally from that bank to keep a good seal, as both cylinders must be the same height. i never assume the factory machined them to the same height anyway.