Cylinder head pushrod hole enlargement

[adamnreeves]

RV8 4.6 Prefix 46D

Yes, my adjustable pushrods again! These things are only 8mm in diameter but seem to call for unprecidented size holes. I have increased the holes to 12mm, leaving 2mm clearance around the pushrod. Erm, Well for some strange reason and I am sure this was the case before I started also, the pushrods were not central in the bores they were all a little close to the rocker assembly side of the hole. Probalby due to geometry of not having shims under my pedestals, the reason for going for adjustables in the first place.
The other issue is that the holes for pushrods 1 and 2, i.e. the exhaust valve ones at the front of the engine seem to be quite close to the front edge of the hole. This does not seem to be the case for pushrods 15 and 16, i.e. the rear most post rods, which seems strange because aren't the heads symetrical?

I thinking of boring out the pushrods holes to 13mm to compensate.

Regards,
Adam.

[adamnreeves]

I spent about 7 hours enlarging the pushrod holes today. About 4 hours of this was spent making a special table to position the head exactly where I wanted it under my pillar drill. I also had to bolt my pillar drill down and make a platform for it also. I had to change the centre line of the holes 40thou closer to the rocker shaft pedestals so that the pushrod are centrally located in the holes. The push rod holes around the central and outer most inlet ports had to remove about 20thou of material. The final bore is 13mm. I think if I ever needed to go to heavy duty adjustable pushrods then then I think I could fit without anymore boring! Overall a good result, would I embark on this again, erm. let me think, NO! However, if I had sent the heads away then I would still have had the off bore issue I feel.

[HairbearTE]

Glad you got it sorted Adam. I know who to see when i need a useful jig for this job now

[adamnreeves]

Indeed. Well jig is probably too elobrate a word for it but certainly would be easy to make into a proper jig with some more bits of wood! I drew lines where to position the head and clamped it down.

Glad you got it sorted Adam. I know who to see when i need a useful jig for this job now

[stevieturbo]

Of the heads I had....As a matter of course, Real Steel said they always enlarge the holes.
I had been using 3/8" pushrods with their heads.

After I destroyed that engine I bought a new pair of stg 3 heads from Rovercraft.

I assumed these would ahve the holes enlarged, being their top spec head.....

Bad assumption.

So, what to do ? I bought the smaller pushrods, and same as you they fouled. I tried to drill etc, but as you discovered its a PITA.

I ended up with most clearing, and a few ever so slightly skimming past the alloy of the head. As I had no proper jigs etc, I just left it at that.

So, if you are buying heads, just make sure you specify larger pushrod holes from the start !!!

[adamnreeves]

Yep I assumed that the holes were enlarged also because they were 10mm in the first place. Also I intentionally went for the 5/16" adjustable pushrods as they were described as the holes might need enlarging whereas the 3/8" were described as pushrod holes do need enlarging.

- Adam

Of the heads I had....As a matter of course, Real Steel said they always enlarge the holes.
I had been using 3/8" pushrods with their heads.

After I destroyed that engine I bought a new pair of stg 3 heads from Rovercraft.

I assumed these would ahve the holes enlarged, being their top spec head.....

Bad assumption.

So, what to do ? I bought the smaller pushrods, and same as you they fouled. I tried to drill etc, but as you discovered its a PITA.

I ended up with most clearing, and a few ever so slightly skimming past the alloy of the head. As I had no proper jigs etc, I just left it at that.

So, if you are buying heads, just make sure you specify larger pushrod holes from the start !!!

[bluepuggsy]

had the same prob. drilled out to 13 mil then had to take a file to the holes next to ilnlet ports as rods were fouling the inlet casting.

[adamnreeves]

The outer inlet ports rather than the centre ones. Yes I thought maybe it was due to drill wandering when enlarging but looked at it very closely and decided it was more to do with the valve train geometry. I found that the front pushrods seemed closer to the inlet ports than the rear ones did. Knowing the heads are not sided this fruther supported my conclusion.

had the same prob. drilled out to 13 mil then had to take a file to the holes next to ilnlet ports as rods were fouling the inlet casting.