[Skip Fix]

What is the next size up from stock if the caps need new dowels? My machinist mentiuoned knurling the stock ones, but I've only heard here of going oversize.

I assume the porcess is pulling the factory ones, torquing caps then using a reamer to get a press fit for the oversize.

[cgeise]

I have been told small block chev head dowels are a few thou. bigger -- although I have never checked for sure.

[BVR421]

Are you thinking the cap holes are getting egg shaped? or do the pins themselves distort? how do you know they are bad? is it obvious or is there another more subtle check? Ive never really thought about it as Ive never seen signs of cap walk or chatter.
I believe the Pioneer catalog has a number of dowel pins but I cant remember if they are listed with measurements or only application.

I always thought it was interesting that the factory thought the cap location method would be more secure using split dowels in the W-72 motors.

[Robert Williams]

Jim Butler sells them, i have some from Butler but haven't used them yet.

[Nine Ninety]

Skip Fix

Maybe its just me, but it would make me wonder if I had my parts at the right machine shop if they mentioned that they wanted to knurl the main cap dowels.........

The fitment and alignment of the caps (as established by the dowels) is critical to engine/bearing life....... If they don't know how or what size dowel to use, maybe you should consider having someone else that has the experience in dowel replacement.

Ron

[Bermuda Blue]

i would never knurl a dowel pin. How would you control the diameter (which is critical). Not mention that they are very hard and probably difficult to knurl. Any machine shop supply house will have the appropriate dowels. Overlooking elongated main cap dowel holes is typical, resulting in bearing failure. I recommend installing oversized dowels and an new machine shop.

[Skip Fix]

I was mainly wanting to be an educated consumer. These guys have done all my Pontiacs for a number of years. I think this is something they haven't had to do before.The block needs some new dowels on the front journal and an align bore. Finding shops that can do the 3.25 of a 455 are few here and in Austin.

I'll inquire more on getting it done.

[Robert Williams]

The pins i got from Butler are .312" x 1.000" long for main cap and cyl head.

[Chris Petersen]

I dont know where my machinist got them, or what size ether, but he told me they were longer so they would sit farther in the cap to make them more stable. Dont know if this is true.

[J.C.you]

careful fitting and assembly required or pop goes the cap, or worse, pop goes the block!

[Jack Gifford]

Ditto on some things said above:
Knurling of standard dowel pins is not an option. Even a "soft" S.S. dowel pin is about 40 Rockwell C hardness- standard alloy dowel pins act to me like at least 50 Rockwell C. Knurled pins wouldn't be advisable here anyway.
Competent machinist would be familiar with over-sizing them.
Line-bore or line-hone should follow re-dowelling, with a few rare exceptions.

I differ with one statement:
A longer pin adds nothing to their function, which is to locate the cap at the parting line with the block. No reason to use other than the shortest available pin- usually 3/8" or 1/2" long.

A few personal opinions from over-sizing a number of main cap dowels on Pontiac V8's:
5/16" dia. pins are available in both .001" and .002" oversizes, if you don't need to correct more than .002" of "slop".
There's plenty of "meat" around the holes to go to a 3/8" pin, if necessary.
To minimize the material removal before and during line-hone/bore (amount milled from caps, and thus the amount removed during hone/bore), I like to make a mandrel (with an OD very close to the ID of the bores) to locate the caps while bolting them onto the block for re-dowelling.
After drilling to within a few thou of the pin diameter (or no drilling, if .001" or .002" oversize pins), I like to use a hand reamer (spiral-flute is better, but straight-flute is okay) to carefully finish the holes- to a light press fit in the block, and a snug slip fit in the cap. Most straight reamers are very slightly undersize on about the first 1/4"-3/8", so that sometimes after reaming with the cap in place, you get the desired fit in both block and cap. If not, you can "tweak" the block and cap hole sizes with the reamer after unbolting the caps.
For do-it-yourselfers, McMaster-Carr is a good industrial-supply house, with no minimum order, except for minimum-package quantities.

[74t/a]

Pontiac dowels measure slightly smaller than 5/16". A std. dowel is .312 and is larger.

The other thing I have found with the stock dowels is they are soft. They are usually a pain to remove too because they want to crush instead of the dowel pin puller gripping. The other thing I have found is they have a very generous chamfer on the top, combined with the fact that they are driven it way too far, and it does not locate the cap very well.

You may need to ream the holes to sized before you install the dowels. If the fit is too tight, it may crack the edge off the cap.

[Half-Inch Stud]

We used sections of Valve Stem to re-dowel my Steel Caps. HIS

[KEN CROCIE]

Std.Pontiac dowels are .309. Common dowels are .3125(5/16) and you can get them anywhere. I've been doing this for years and highly reccomend replacing the dowels when a line bore is indicated, otherwise the cap will never locate in the same place twice.

[Half-Inch Stud]

So 5/16 Valve Stems make an excellent source for Lengthened Dowels HIS

[Skip Fix]

Thanks guys for all the info.

[theoldone]

Originally Posted by Skip Fix

What is the next size up from stock if the caps need new dowels? My machinist mentiuoned knurling the stock ones, but I've only heard here of going oversize.

I assume the porcess is pulling the factory ones, torquing caps then using a reamer to get a press fit for the oversize.

there are (2) stock Pontiac dowel sizes- small and large

some 2-bolt main blocks have smaller dowels than 4-bolt main blocks

I converted a 2-bolt 455 to 4-bolt by using 428 4-bolt caps- the block was already drilled/tapped for 4-bolt caps, it was a 1971 455 block

the 2-bolt dowels were smaller than the 428 cap dowel holes- I had to pull all the dowels out, have them drilled on a bridgeport to the correct size, then buy the larger dowel pins, cut them to size on a small precision saw, debur them, and pound them into the block lightly with a small hammer

then attach the caps and align hone the block

if you change the dowel pins, you have to align hone the block- it will affect the cap placement on a Pontiac

[Tim Corcoran]

I know this is an old topic but I have been looking for some info on this subject. I am replacing the dowel pins on my 400 stroker that I am currently building. I am also converting to 4 bolt mains and the block is already drilled and tapped for 4 bolt mains. The original pins in the block were pretty tight and were difficult to remove with a dowel pin removal tool but I was able to get them all out. The reason I am changing them is they were not very far above the deck and have a rather large taper and I am not comfortable using them like that, the #1 cap even rocks a little because of the lack of engagement. The original pins mic out at .3.009 and the new pins are .3125 by 7/8" long. Since you want the pins in the block to be an interference fit and the caps to be a slip fit you need a little clearance to compensate for hole in block to hole in cap will never line up perfect. My plan is to ream the caps to .3135 and ream the block to 3.115. If I do this the pin holes in the block will be .001 smaller than the new pins and the hole in the caps would be .001 larger than the new pins. I have located a .3135 reamer but having difficulty finding a .3115 reamer. If you are a machinist or have experience with this please chime in with your thoughts.

[steve25]

If your running a cast main cap then even with stock size pins the hole for them in the bottom of the cap should be chamfered to reduce the chance of a crack starting .

[Skip Fix]

Tim and old post! So something I came across on having a shop convert a 400 block to 4 bolt splayed caps. Two of the caps are tight with the factory dowels another loose. All have the same dowel OD and the same hole ID in the caps. I think there must be a slight difference in placement of the dowels in the block vs dowel holes and bolt holes in the caps to give some interference on the tight ones vs the loose one. Will reaming the block or cap holes for bigger be a hair offset? Of course i found out when checking the block after line bore cam bearings installed. etc. I figured the machine shop would check for a tight fit before all that-nope.

Now is it enough to pull the cam bearings as a new line hone will potentially send some trash up the feed holes on a street/strip build? Or will the splayed bolts do good enough.

There are some folks that sell longer stock diameter dowels for a little more engagement.