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Relief to the bore

Started by _Muzzy_ , May 14 2016 10:20 AM

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28 replies to this topic

#1 _Muzzy_

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Posted 14 May 2016 - 10:20 AM

At what point do you need to relieve the bore
I'm running a 93mm bore
1.740" intake
And 445" lift base on 1.5:1 rockers or .480" if I use 1.6:1 rockers
The head has had .040" removed from it
I'm stuffed if I can work it out on measurement and depth gauge, it's not looking good without .100" or relief

Have looked on the web and get mixed answers?

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#2 _repco_

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Posted 14 May 2016 - 10:37 AM

Hi Brian. You need to start by putting the head gasket on the head and the block and then scribe around the fire ring of the gasket. Then you will see how things look and also have a measurement to work with. Do not go lower than the top ring at tdc. Phil.

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#3 _Muzzy_

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Posted 14 May 2016 - 10:53 AM

Hi Brian. You need to start by putting the head gasket on the head and the block and then scribe around the fire ring of the gasket. Then you will see how things look and also have a measurement to work with. Do not go lower than the top ring at tdc. Phil.

I have done this on the head , and taken measurements not on the block will do no keep me surfing, I do understand the max relief would be no more than 2-3mm as a scallop

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#4 _Muzzy_

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Posted 14 May 2016 - 10:56 AM

I have done this on the head , and taken measurements not on the block will do no keep me surfing, I do understand the max relief would be no more than 2-3mm as a scallop

Auto correct gotta live it,

I will measure the block with head gasket on it and scribe, Not sure where surfing comes into it LOL

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#5 _repco_

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Posted 14 May 2016 - 10:59 AM

You need to mark the block. and machine the relief out to the scribed line.

Bloody wax head.... LOL

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#6 N/A-PWR

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Posted 14 May 2016 - 11:25 AM

Keep valve clearance to a minimum as the compression will lower.

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#7 _oldjohnno_

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Posted 14 May 2016 - 03:13 PM

At what point do you need to relieve the bore
I'm running a 93mm bore
1.740" intake
And 445" lift base on 1.5:1 rockers or .480" if I use 1.6:1 rockers
The head has had .040" removed from it
I'm stuffed if I can work it out on measurement and depth gauge, it's not looking good without .100" or relief

Have looked on the web and get mixed answers?

It's unlikely you'll have to do anything. Sit the head on the block with a gasket and with an intake and exhaust valve fitted to one cylinder with no springs. Clothes pegs are an impressively hi-tech way of holding the valves up while you drop the head on. Use verniers to measure the distance from the tip of the stem to the top of the guide with the valve closed then push the valve down til it touches the block and measure it again. Hopefully the difference between the two measurements will be equal to or greater than the valve lift plus a bit for clearance, say 0.040". Don't hack away at the top of the bores unless you have to, but with small cams it usually isn't a problem anyway.

Edited by oldjohnno, 14 May 2016 - 03:15 PM.

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#8 _Muzzy_

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Posted 14 May 2016 - 03:59 PM

It's unlikely you'll have to do anything. Sit the head on the block with a gasket and with an intake and exhaust valve fitted to one cylinder with no springs. Clothes pegs are an impressively hi-tech way of holding the valves up while you drop the head on. Use verniers to measure the distance from the tip of the stem to the top of the guide with the valve closed then push the valve down til it touches the block and measure it again. Hopefully the difference between the two measurements will be equal to or greater than the valve lift plus a bit for clearance, say 0.040". Don't hack away at the top of the bores unless you have to, but with small cams it usually isn't a problem anyway.

I measured the bore to gasket and have 2mm I measured the gasket to intake valve and have 3.50 mm so I should be right, however I will do that, sit the head on and check depth ,
Thanks all

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#9 _STRAIGHTLINEMICK_

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Posted 14 May 2016 - 04:35 PM

Simplest way to check is to remove a couple of valve springs and fit any light spring that can easily be compressed, put the head on the block without gasket and push both valves down to their max lift to see if everything clears. The head gasket thickness will be your safety margin.

Edited by STRAIGHTLINEMICK, 14 May 2016 - 04:36 PM.

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#10 _STRAIGHTLINEMICK_

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Posted 14 May 2016 - 06:03 PM

Forgot to mention that on some engines the valve to block clearance can vary between cylinders due to production tolerances or the lack of . My engine was an example of this.

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#11 warrenm

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Posted 14 May 2016 - 11:26 PM

As oldjohnno said, I don't think you'll have a problem with a cam with small lift.

If you had another .100" valve opening you would be getting closer to the bore.

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#12 N/A-PWR

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Posted 15 May 2016 - 12:09 AM

Just for information my 9 port has 100 thou shaved from it,

which is down to the witness marks,

and with 1.625" diameter inlet valves,

they are only 5/16" from the deck on the port side,

and 1/2" from the deck on the plug side.

The closest to the bore's edge is half way between the two.

In my case, the valves misses the bore in a 208ci

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#13 _STRAIGHTLINEMICK_

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Posted 15 May 2016 - 08:49 AM

As oldjohnno said, I don't think you'll have a problem with a cam with small lift.
If you had another .100" valve opening you would be getting closer to the bore.

Yep usually you'll be ok but it needs to be checked when youre getting close to .500 lift.

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#14 N/A-PWR

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Posted 15 May 2016 - 01:18 PM

Hello Brian,

This is roughly accurate, and used a piece of paper to press against the head chamber No.6,

then pressed the same piece of paper against the No.6 of the 208ci block,

measured the actual sizes to corresponding points,

then transferred them to the photo of the 186ci head chamber.

I see clearances to allow your valve in a 93mm bore,

but to be sure I would put a rolled up plasticine 6mm thick hotdog,

along the bore edge and cover with a piece of glad wrap over it to stop sticking,

put 3mm spacers both ends of the block,

place the head on and lower the valve the full lift plus 1mm,

take head back off and check the impression if any.

If there were to be any machining, mark the block where the imprint is,

then get a milling machine set up to the angle of the valve,

and flycut 2mm diametre bigger than the valve,

to the depth of the imprint from the head face (glad wrap).

Someone will be able to explain any further information, as this is just an idea to start.

Paper press print chamber and bore.jpg 217K 1 downloads

208ci 186 head 100 thou shaved.jpg 134.12K 3 downloads

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#15 _rich243_

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Posted 15 May 2016 - 03:50 PM

Do the 1.6 ratio rockers muzza is using make much difference?

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#16 _Muzzy_

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Posted 15 May 2016 - 04:31 PM

Do the 1.6 ratio rockers muzza is using make much difference?

I don't know, I haven't brought them as yet, going off the SBC figures and engine size I reckon about 8-10 HP on a 202, I m happy to here others who have them

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#17 _rich243_

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Posted 15 May 2016 - 05:03 PM

I meant in regards to needing to reliev the bore. But it seems youve factored that in to your lift figures so all good.

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#18 N/A-PWR

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Posted 16 May 2016 - 01:30 PM

Oldjohnno has valuable information here:-

http://www.oldjohnno.id.au/page16.html

http://www.oldjohnno.id.au/page14.html

and

here is your answer:-
http://www.gmh-toran...xu1-valve-size/

Edited by NA-PWR, 16 May 2016 - 01:43 PM.

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#19 _imperial engines_

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Posted 21 May 2016 - 05:57 PM

Hi, you say you are running a 93mm bore, what pistons do you use?

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#20 _Muzzy_

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Posted 21 May 2016 - 09:09 PM

Hi, you say you are running a 93mm bore, what pistons do you use?

Off set bowl ACL duralites II ( NOS)

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#21 _Muzzy_

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Posted 11 June 2016 - 01:10 PM

Well did all the measurements today
Very close
With a used gasket fitted
The exhaust are all fine
Intakes vary, however 3 are sitting at 020" clearance, they hit just on the edge of the bore, where the machined taper is, the other 3 hit, but clear the taper and hit the Bore that gives me more than enough with 1.5:1 rockers

Mm mm
I think I will touch up the taper with my little Dremel, I'm not happy with .020" clearance

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#22 _Muzzy_

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Posted 11 June 2016 - 04:02 PM

Here a couple of pictures of the blue mark on the cylinder taper

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image.jpg 158.44K 2 downloads
image.jpg 120K 5 downloads

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#23 S pack

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Posted 11 June 2016 - 05:55 PM

Wow the top of this piston bowl area looks like course sandpaper but the adjacent piston looks to be machined. Is the grainy look an optical illusion?