Inline-boring Steel Main Caps

[JonnyPlus2]

Hi All;
In his book Tuning Twin Cam Fords, on pg. 34 David Vizard states "Having acquired these (steel main caps), they must be inline-bored to 0.015" oversize to accommodate the Vandervell competition bearings".

My machinist thinks that the oversize bore is unnecessary to ensure that the mains are true. Does anyone know why Vizard spec'ced 0.015" oversize? BTW the main caps are from QED and I used their socket head cap screws.

Thanks much.

Best regards,
Jonny

[types26/36]

I don’t know the answer to your question but maybe the “Vandervell competition bearings” have a thicker bearing material so consequently the outer diameter is larger ? …….just a thought

[Craven]

When you line bore, material is removed from both cap AND block.

[promotor]

Hi All;
In his book Tuning Twin Cam Fords, on pg. 34 David Vizard states "Having acquired these (steel main caps), they must be inline-bored to 0.015" oversize to accommodate the Vandervell competition bearings".

My machinist thinks that the oversize bore is unnecessary to ensure that the mains are true. Does anyone know why Vizard spec'ced 0.015" oversize? BTW the main caps are from QED and I used their socket head cap screws.

Thanks much.

Best regards,
Jonny

To remove a set of caps (or start with a block without them) and fit new caps and retain the original bore size and position requires either a lot of luck that the size and alignment of replacements are absolutely spot-on, or a very skilled machinist to bore the replacement mains caps without taking any more material out of any surface of the original main bearing (half)bore in the block, and make round holes. A couple of thousandths of an inch here or there will make out-of-round holes.
So with that method and its problems it is sometimes wise to make a new bore slightly oversize to make sure the holes are perfectly round and where there aren't any spots where the machining tool has "hit or miss" in various places due to set-up / alignment issues.

The other way to keep the original bore size is to machine /surface grind the platforms where the mating faces for the mains caps sit (the faces on the block, not on the caps) in the block down by a small amount (perhaps 0.005" - 0.010" or so, as a guess), then machine the mating faces of the mains caps themselves down slightly, making the caps shorter, and then carefully set-up to machine an original size hole - taking material from both the original block bearing bores and the replacement caps - this method means the main bearing bores will be slightly higher in the block once finished (ie towards the head and maybe even moved sideways one way or another). In doing so you are moving the centre line of the crank slightly in the block, but hopefully no more than a negligible amount so as not to affect crank position and therefore how much room the pistons have before being proud above the block face (and you then start to run into other issues). If it doesn't go too well the crank centreline may move more than a small amount.

The question you may want to ask your machinist is how round will the holes be and how far will the crank have moved etc in the block after the centreline is changed, if they choose that method.

There are people who can do it either way, but making perfectly round and true to size holes / bores once finished is another story.

[JonnyPlus2]

Thank you all for taking the time to reply, especially @Promotor for explaining this tricky subject clearly.

Best regards,
Jonny

[rgh0]

I have had multiple blocks line bored in the way described by promotor. The cap seats on the block are milled about 5 thou and the caps milled about 5 thou undersize and then the assembly is line bored back to the orginal size to use standard O/D bearings with the correct crush. You may find that the steel caps you have are already undersize in preparation for line boring

regards
Rohan

[promotor]

Thank you all for taking the time to reply, especially @Promotor for explaining this tricky subject clearly.

Best regards,
Jonny

No problem. One more thing - I understand it can be a more difficult job to machine man bearing accurately when using two dissimilar materials ie a cast iron cylinder block and steel caps as one material wants to deflect the cutter more than the other. Of course, that can be solved by a rigid machining set-up but what that actually is will be open to interpretation.

[Craven]

Sounds quite tricky to mill this lot.

[JonnyPlus2]

I shall go with your advice to mill 5 thou, off the block and the caps and then line bore them to standard size. Thanks again for sharing your knowledge and experience, it is much appreciated.

[gjz30075]

After a line bore job, it's best to check the concentricity of the bellhousing to the centerline of the crank. If
it's out too far (and I'm not sure what "too far" is), you'll have trouble shifting because the gearbox input
shaft will be constanting pressing against the crank bushing/bearing. I'm guessing no more than .010",
but maybe Promotor can chime in on this.

[JonnyPlus2]

Hi Greg,
I haven't dialed in my bell housing yet so all is good!
Best,
Jonny

[promotor]

After a line bore job, it's best to check the concentricity of the bellhousing to the centerline of the crank. If
it's out too far (and I'm not sure what "too far" is), you'll have trouble shifting because the gearbox input
shaft will be constanting pressing against the crank bushing/bearing. I'm guessing no more than .010",
but maybe Promotor can chime in on this.

It's certainly a consideration and the crank centre line will be in a different position relative to the gearbox. How much is too much though? I can't answer that. I can say that the relationship / positioning of the crank and gearbox is important - Ford made sure everything is concentric by using dowels to mate the bellhousing and block together. Looking at how good the machine work is elsewhere in the engine and gearbox I'd say they are going to have good tolerance on concentricity from the factory.
I'd say 0.010" movement on the crank although not ideal would mean to just keep a check on gearbox noise and quality of gearshift - if anything changes from "normal" then that'd be the first thing to check.

[gjz30075]

Thanks Promotor. I put a T5 in mine and Tremec calls for no more than .005. I had to remove
one of the dowels to get the desired movement to get it close. I managed to get it to .002, then
I drilled through the bellhousing and block in two places and pinned it with hardened roller pins.

[2cams70]

They are actually good points being raised here about the shift in the crankshaft centerline. Not only is the crankshaft centerline being moved relative to the gearbox input shaft front and rear oil seals and compression ratio are also being affected. My engine had been so machined and when it came to sourcing con rods it was quite difficult to source them at standard length which I had to do because of the increase in CR caused by this rather than use the more common Cosworth length which is .026” longer. The old school 0.015” oversize tunnels is the best method. Problem is there aren’t many matching bearings available these days that are 0.015” oversize on the OD

[JonnyPlus2]

Just got off the horn with my engine guy. The shop that is doing the actual machine work on the block does not line bore the main caps. Instead (gesticulates wildly) they use a very sophisticated computer controlled milling machine of some ilk that can cut the steel main caps to match the radius of the main bearings. My engine guy assures me that we will get the mains and caps round to tight tolerances.

Worst case scenario is that I will be forced, forced to buy an alloy 2 litre block.