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Valve guide interference fit: Bronze vs cast iron guide?

General engine tech -- Drag Racing to Circle Track

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Nut124
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Valve guide interference fit: Bronze vs cast iron guide?

Post by Nut124 » Thu Jun 25, 2020 12:32 pm

I'm replacing the factory cast iron guides in my Fiat twincam aluminum head with new bronze guides.

Is the amount of interference for bronze any different from what is specified for cast iron?

The service manual calls for 0.0008 to 0.0026 interference for the cast guides.

Any help would be appreciated.


Nut124
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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by Nut124 » Thu Jun 25, 2020 10:12 pm

Thanks. I plan on baking the head in 400F oven while cooling the guides in a freezer.

I made a guide driver that drives on the seal shoulder rather than the thin top of the guide.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by BILL-C » Fri Jun 26, 2020 12:29 pm

Want your seats to fall out too? I would do .0015-.002 room temp press fit and pre-heat head to 250* F
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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by BillK » Fri Jun 26, 2020 1:38 pm

Nut124 wrote:
Thu Jun 25, 2020 10:12 pm
I plan on baking the head in 400F oven
That should ruin it real good :(

I never go over about 160 degrees and put the guides in the freezer and they go right in. I use some CMD Extreme Pressure Lube in the guide bores.
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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by GOSFAST » Fri Jun 26, 2020 4:59 pm

I would just add this, I would not try to "bang" the old cast guides out, you run the risk of "tearing up" (galling) the guide holes??

I would "core-drill" most of the old guide out and just leave a step at the bottom to drive out the remaining shell! In other words don't "core-drill" all the way through the guides.

Thanks, Gary in N.Y.

P.S. Then follow Bill K's info above, works really well. The only difference we do here is put the new guides in dry-ice instead of a freezer, but the freezer will work!
Sept. 2019 - Drag-Week Winner - New York Street Ride 7.23+ @ 196+ @ 3800#+

Nut124
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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by Nut124 » Fri Jun 26, 2020 7:13 pm

BillK wrote:
Fri Jun 26, 2020 1:38 pm
Nut124 wrote:
Thu Jun 25, 2020 10:12 pm
I plan on baking the head in 400F oven
That should ruin it real good :(

I never go over about 160 degrees and put the guides in the freezer and they go right in. I use some CMD Extreme Pressure Lube in the guide bores.
Thanks for the feedback. Yes, 400F is likely too hot. I ended up doing 300F per the link posted by dannobee. The guides were in the freezer. Used some oil. They still required some 10 stiff blows per guide to go in. Per my measurements, interference was just about 2 thou at room temp. I used a custom spacer around the guide. The shoulder on the driver tool hit the spacer and stopped when guide was at proper depth.

I fabricated reamer center cones that center on the valve seat. Reamers coming tomorrow.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by BillK » Fri Jun 26, 2020 7:54 pm

Nut124 wrote:
Fri Jun 26, 2020 7:13 pm

I fabricated reamer center cones that center on the valve seat. Reamers coming tomorrow.
Not sure if that is a good idea or not ?? The reamer is probably going to follow the guide no matter what and if you try to force it off to one side with the cone it is liable to end up making an oblong hole ? I would just ream the guide, finish hone it and then cut the seat to match the guide.
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Nut124
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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by Nut124 » Fri Jun 26, 2020 8:11 pm

BillK wrote:
Fri Jun 26, 2020 7:54 pm
Nut124 wrote:
Fri Jun 26, 2020 7:13 pm

I fabricated reamer center cones that center on the valve seat. Reamers coming tomorrow.
Not sure if that is a good idea or not ?? The reamer is probably going to follow the guide no matter what and if you try to force it off to one side with the cone it is liable to end up making an oblong hole ? I would just ream the guide, finish hone it and then cut the seat to match the guide.
Bill, thanks for the advice. Are there different kinds of reamers? do some follow the bore better than others? Mine are coming from CylinderHeadSupply. I asked them if these were made for use in a hand drill or in a trued machine. They said trued machine. When I put the centering cone on the seat, a 5/16 drill bit drops right in to the guide. I have drilled many a crooked hole. When/if a bit catches, the drill will twist off center. I feel like this is better than doing it by hand alone. Two old guys at a local shop told me that they just chuck the reamer and do it by hand, which i found a bit odd.

Thoughts?

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by dannobee » Fri Jun 26, 2020 8:55 pm

When the stuff really mattered, (i.e., expensive race heads or my own stuff), I'd ream by hand, then check concentricity. I'm sure that others have a preference.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by MotionMachine » Fri Jun 26, 2020 9:27 pm

Ream it if you feel you must, but then have it properly sized by a shop that has a rigid guide hone.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by Nut124 » Fri Jun 26, 2020 10:17 pm

While I do like working on stuff myself, and I'm pretty good at it, having learned at the Bubba school of machining for 30 some years, I would not do this if I had access to competent shops with reasonable turnaround times.

There are two shops within 40 miles that I know of. One builds race engines and is friendly, but cannot take new work or offers 4 weeks which is actually more like 6 weeks. The other did not have the tooling for bronze guides in my size.

If I get the guides installed and reamed to size they would do the 3 angle seat cut for me, which I cannot do myself, or can I?

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by dannobee » Fri Jun 26, 2020 11:44 pm

Well, you could, but the tools aren't cheap and you'll have more money into even buying the cheap tooling than just sending the head out.

But if you really want to, and time is of the essence (considering their turnaround time), call Neway manufacturing and tell them what you're working on and they'll set you up. There is a learning curve, but with the pilot, cutters and a concentricity gauge, it's doable. I always kept a cutter kit in the race trailer just in case something went south. Like I said, there's a learning curve, but with the proper gauges and going slowly while checking your work, it'll get you back on the track. It's no Serdi or Rottler SG, but it will give acceptable results.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by AC sports » Sat Jun 27, 2020 1:13 am

I dream by hand then run a small flexhone through the guide to improve surface finish. Dont overdo the flexhone though. Depending on the guides you got.....I find that some close up more than others or rather are a smaller ID to start with. 2 thou fit on the od is typical of the fiat tc aftermarket guides. They are also avail in oversize.

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Re: Valve guide interference fit: Bronze vs cast iron guide?

Post by Sparksalot » Sat Jun 27, 2020 1:32 am

There two general types of reamers: hand reamers and chucking reamers. Which type did you buy?

Hand reamers are tapered with a gentle taper to enter the hole and always have a square driving feature on the shank end. They are driven by hand with a tap wrench. As the name implies they are meant for hand reaming jobs and will self align with the hole.

Chucking reamers are designed with only a chamfered cutting feature on the end and are meant to used in rigid machines with spindle axis in alignment with the hole. Their shanks are completely cylindrical. Do not use a chucking reamer without a rigid and aligned setup.

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