Machining Liners

[woodward]

Hi

I’m rebuilding my TR4 engine at the moment with 89mm liners, trgb piper yellow cam and stage 2 head. I read somewhere on the forum (though can’t find it now) that I should machine a 45 degree angle in the top of the liners for best results.  Is this something I should ask the machine shop to do?

Also when I dropped the head off I was asked what compression ratio I wanted.  What is the common wisdom?  The car will be rallied/hillclimbed but I drive the car to/from events so am so looking for something which will be ok to drive in traffic.  My thoughts at the moment are to leave the CR more or less stock as I believe I’ll get a little bit of an increase with the 89mm pistons anyway? 

Thanks

Myles

[Motorsport Mickey]

More information required Myles, head gasket ? what deck height do you plan for the pistons to be machined at ? 

Forget about the planned little bit of increase from bigger pistons, you need to plan what do you want to run at, you can't just leave it all fall into place and hope for the best. If you don't want to use additives current thinking is that about 10.3 compression is about the limit for our current petrol. These figures all depend upon what headgasket will be used and because of that what the pistons need to deck at, all which will alter compression ratio. Although you only plan low grade competition without needle bending revs I would definitely have all the rotating masses balanced for better pickup and smoother running. The 45 degree angle you refer to on top of the liner is a reference I think to the chamfers which need filing or machining into the liner tops after the head work is completed and their chamber outlines transferred onto the liner tops so that the overlapping inlet valve has somewhere for the charge to go other than being sucked through the valve and bashing it's head upon the shoulder formed by the liner. This all well known and is as practiced over the last 40 years amongst others by Kastner

.   

Edited February 7, 2022 by Motorsport Mickey

[AndrewMAshton]

Hello Myles, have just gone through what you are planning, excellent article by Revingtons on calculating your compression ratio,

https://www.revingtontr.com/information-sheets/is0064-compression-ration-calculation , you will have to find the link on this page, you also need to measure the head volume, I used a sheet of Perspex with hole drilled and sealed with grease I then used a medical syringe to measure the volume, I used methylated spirits, bit cleaner,

My CR is 10.1 with 89mm pistons,

Good Luck, Andrew

[ianc]

At his request, I supplied the updated article to Neil last year, and am glad that Andrew found it useful.

As it happens, with my son, Alex, I carried out a compression test on 4VC yesterday, obtaining 180-185 on all four - this tends to confirm my estimate of a CR of about 10.0 on the 2138 (Correction, 2238 as have 88mm pots) engine.

Ian Cornish

Edited February 10, 2022 by ianc
Correction to engine size

[BlueTR3A-5EKT]

9 hours ago, ianc said:

At his request, I supplied the updated article to Neil last year, and am glad that Andrew found it useful.

As it happens, with my son, Alex, I carried out a compression test on 4VC yesterday, obtaining 180-185 on all four - this tends to confirm my estimate of a CR of about 10.0 on the 2138 engine.

Ian Cornish

Interesting bit from Barney Gaylord, MGA Guru here about the relationship or lack of, between CR and pressure.

https://mgaguru.com/mgtech/power/pp105.htm

Edited February 9, 2022 by BlueTR3A-5EKT

[woodward]

Thanks all for your help on this topic.  I had another read of Kastner's book and the technicalities material to help understand the issue.  Given that I'm not building a race car I've come to the conclusion that I won't need to machine the liner tops.

One other question though - I still need to decide on the CR.  I'm not planning to deck the pistons but I do have a choice on head gasket.  What is the recommended head gasket for a non race spec engine with 89 mm pistons?  Composite or copper?

Thanks

Myles

[Lebro]

89mm means you can't use the standard composite "86mm" gasket. several suppliers do one intended for 89mm - TR shop, Revingtions, Racetoracions etc.

They are rather more expensive than the standard one.

Bob

[Motorsport Mickey]

Myles,

If you go for the 89mm piston without making the cutouts in the liner tops you are throwing away a lot of the 89mm advantage, we even do those cut outs for 87 and 86 mm pistons builds. The valve in their bathtub pockets near enough overlap the liner dia which constricts the incoming gas flow on even standard engines, the chamfering of the liner top surface can easily be carried out by hand using a file and will give major benefits to the gas flow. Kastner and the rest of us did it to release the extra power when racing, but even if used on a road engine you will benefit from removing the obstruction caused to the gas whistling through the small valve gap and then being trapped against a 90 deg wall !

As always only as suggestion, your car...your choice.

Mick Richards