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BBC Guideplate clearance question

General engine tech -- Drag Racing to Circle Track

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Bob Hollinshead
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BBC Guideplate clearance question

Post by Bob Hollinshead »

I'm using comp cams 3/8" guideplates and the slots have .025" of pushrod clearance that's giving a lot of allowable tip movement. Are there other options and or how much clearance in the slot should be there? The smallblock stuff I've done in the past never seemed to have as much clearance.
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Re: BBC Guideplate clearance question

Post by ProPower engines »

Bob Hollinshead wrote: Fri Jun 12, 2020 12:45 pm I'm using comp cams 3/8" guideplates and the slots have .025" of pushrod clearance that's giving a lot of allowable tip movement. Are there other options and or how much clearance in the slot should be there? The smallblock stuff I've done in the past never seemed to have as much clearance.
Dart sells a nice adjustable guide plate set. They fit the push rods closer then the comp stuff does and allows for better alignment of the roller tip to the valve tip center.
The guide plates are sold in a set for 1 cyl. head so be ware you must order 2 sets for 2 heads :D
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Re: BBC Guideplate clearance question

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ProPower engines wrote: Fri Jun 12, 2020 2:40 pm
Bob Hollinshead wrote: Fri Jun 12, 2020 12:45 pm I'm using comp cams 3/8" guideplates and the slots have .025" of pushrod clearance that's giving a lot of allowable tip movement. Are there other options and or how much clearance in the slot should be there? The smallblock stuff I've done in the past never seemed to have as much clearance.
Dart sells a nice adjustable guide plate set. They fit the push rods closer then the comp stuff does and allows for better alignment of the roller tip to the valve tip center.
The guide plates are sold in a set for 1 cyl. head so be ware you must order 2 sets for 2 heads :D
Thanks for the info, I've seen those adjustable plates and they look like a big help with alignment. Any idea how wide the slot is? I see AFR and Comp sells the adjustable versions too. I'd like to get this dialed in with a little closer tolerances so hopefully full chaos doesn't happen at higher rpm
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Re: BBC Guideplate clearance question

Post by ProPower engines »

It also depends on who's push rods your going to use. Some are slightly smaller OD then others.
I always measure the guide slot 1st then order push rods accordingly
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Re: BBC Guideplate clearance question

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ProPower engines wrote: Sat Jun 13, 2020 12:37 am It also depends on who's push rods your going to use. Some are slightly smaller OD then others.
I always measure the guide slot 1st then order push rods accordingly
It would be nice if the guideplate manufacturers listed the width of the slot, with .025 clearance on these ones from Comp the rocker has enough movement to move sideway on the valve tip a third of the width of the roller. How much variance in diameter have you seen on pushrods?
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Re: BBC Guideplate clearance question

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Bob Hollinshead wrote: Mon Jun 15, 2020 1:41 pm
ProPower engines wrote: Sat Jun 13, 2020 12:37 am It also depends on who's push rods your going to use. Some are slightly smaller OD then others.
I always measure the guide slot 1st then order push rods accordingly
It would be nice if the guideplate manufacturers listed the width of the slot, with .025 clearance on these ones from Comp the rocker has enough movement to move sideway on the valve tip a third of the width of the roller. How much variance in diameter have you seen on pushrods?
Again guide plates are just a stamped out part. The stamping die's can vary from well used to a fresh die. the worn die's will produce a smaller slot where the fresh die's will be bigger.
That is based on typical manufacturing of stamped parts where 1000's of parts are stamped before anyone checks sizing
which seems to continue to be an issue in the industry.

Also like all tubing there is a mill tolerance for diameter and wall thickness. Any tube with a Mil. ( military ) spec will be tighter to size then just the std. tubing but there can be up to a .005-.008 tolerance on wall or OD depending on the tube size being used.
Again its best to order push rods with a known OD after confirming the guide slot width. This will reduce the slop and excess movement of the rocker.
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Re: BBC Guideplate clearance question

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My BBC guide plates were originally 3/8”. I switched to Trend 7/16” on the intakes so I used a 3/8” carbide endmill and opened the slots to 7/16”. I am using 3/8” on exhaust, however I am switching to 7/16” there now too. Will be milling the slots in the same fashion. Like being able to set the clearance width.

Eventually I would like to switch to shaft rockers but the rebuild only has 25 passes on it and I don’t want to pull heads to mill for the Jessel/T&D/Crower stands right now. Just want to race.😃
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Re: BBC Guideplate clearance question

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PmGRacer wrote: Thu Aug 27, 2020 8:22 am My BBC guide plates were originally 3/8”. I switched to Trend 7/16” on the intakes so I used a 3/8” carbide endmill and opened the slots to 7/16”. I am using 3/8” on exhaust, however I am switching to 7/16” there now too. Will be milling the slots in the same fashion. Like being able to set the clearance width.

Eventually I would like to switch to shaft rockers but the rebuild only has 25 passes on it and I don’t want to pull heads to mill for the Jessel/T&D/Crower stands right now. Just want to race.😃
Check with dart.
They have adjustable guide plates for 3/8 and 7/16 pushrods.
They are sold in sets of 4 so you can get 4 sets of each and then use the size needed in the location of the larger push rods
that way all you will need to do is set them in the proper location for roller tip alignment and tighten them up. =D>
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Re: BBC Guideplate clearance question

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I found out there was a big difference in slot width for the 3/8 guideplates and a difference in pushrod diameter for 3/8 pushrods depending on the manufacturer. I ended up going to Manley guideplates and ended up with .009-.011" clearance. The Comp plates were giving as much as .028" clearance and let the rocker tip move around too much it seemed. Almost ready to fire this up so hopefully it works good
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Re: BBC Guideplate clearance question

Post by PmGRacer »

Check with dart.
They have adjustable guide plates for 3/8 and 7/16 pushrods.
They are sold in sets of 4 so you can get 4 sets of each and then use the size needed in the location of the larger push rods
that way all you will need to do is set them in the proper location for roller tip alignment and tighten them up. =D>
[/quote]

I did call Dart, they only sell 3/8” guide plates now.

I did find a local engine shop that had two Comp Cams 3/8” guide plates in stock that I bought. I machined the first one clamping square in the mill vice and milling they slot straight. When I put it on the head and checked the tip alignment it was off considerably. Measuring to difference to the OEM gp the Comp was off about 4 degrees. I rotated the vice 4degrees, milled the slots and put it back in the engine. Now the rockers lined up correctly.

I only used 1 Comp gp to replace the broken OEM one. I then machined the ex slots on the rest of the OEM gps to clear the push rods on the throat. The side clearance is 0.016”. The push rods measured 0.435” and I made the slots 0.450- 0.451.
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Re: BBC Guideplate clearance question

Post by Bob Hollinshead »

I ended up with .009-.011 clearance on all of mine hoping that will work good
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