Help With Engine Rebuild/Larger Head

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MarkMan900
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Help With Engine Rebuild/Larger Head

Post by MarkMan900 »

Hi,

Apologies for most likely going over similar ground to other threads but I did search the forum regarding the 12G940 head and there are so many posts I struggled to get a clear answer.

Basically I am doing some work including a respray to my 1963 Minor and while all the work is going on I am planning to rebuild the engine while the body is in the garage. The engine has always had a bit of an oil leak but it has become unbearable so I planned to at the very least have it all apart, give the whole thing a good clean and replace all the gaskets. However we did a compression test last week and the results were dire (95, 55, 55, 75) so the rebuild is going to be more intensive. We tried the oil in the cylinder trick which brought the compression up by about 20-25psi but still not enough so I am assuming the valves are also shot. In fairness I have run it for 4 years without additive so expected them to be replaced at some point.

I was looking at getting unleaded valves done at a local garage but for not much more than the same price they charged I can get a rebuilt 12G940 head with unleaded valves. I therefore decided to grab a copy of David Vizard's book (great so far although seems to be aimed at more performance than I need) and have been having a look through. I don't need a racer or fire breathing engine but enough power to keep up on the dual carriageway without totally killing fuel economy would be nice.

I am therefore thinking of the following:

- Current 1098 block rebored (judging by those figures I guess this will be a necessity) with the smallest oversize pistons we can get away with.
- Rebuilt 12G940 unleaded head off a mini 1275 (but with the standard rocker assembly off my 1098 head)
- Rebuilt HIF38 carb on an alloy mini manifold
- 1275 Cooper Freeflow 3 LCB Maniflow Exhaust Manifold
- 1.75" or 2" stainless steel exhaust system (not decided which yet)
- Standard 1098 ribbed case gearbox (although this is getting rebuilt too as it also leaks oil and jumps out of 2nd on the overrun)

I just wanted to ask if anyone else had done anything similar or if my ideas were way off the mark for what I'm trying to achieve?

I also noticed that the 12G940 head has a slightly lower volume so will presumably raise the compression ratio of the engine. Is this going to cause a large problem, also would the reboring/oversized pistons negate this effect slightly?

The 12G940 head also seems to mean the exhaust valve overhangs the block so the block needs to be machined to make space which will also presumably reduce the compression ratio slightly too?

If there is another thread where this question has already been answered please let me know, I just couldn't find it amongst the many many posts on the 12G940 head on the forum.

I'm hoping to just up the power enough to improve driveability on bigger roads so I feel like this is the best time to do it, but don't want to accidentally ruin my engine in the process. Any help or advice would be greatly appreciated!
paul 300358
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Re: Help With Engine Rebuild/Larger Head

Post by paul 300358 »

12g940 head fits but you need to ensure that the exhaust valves don't hit the block, I had to sink my exhaust valves 0.040" into the head. You will need also need the sintered type rockers as the standard 1098 rockers don't align with the valves on the 940 head.
Make sure that you use the 1275 head gasket.
The cable operated heater valve won't fit, it hits the battery box, you will have to fit a brass tap type valve or make an extension.
You also need to file the top from the water pump as the head is slightly longer and will rest on top of the pump.
Hif 38 with alloy inlet is a good idea but you do loose the hot spot which is there to allow the choke to be pushed in quicker. I actually wished that I had fitted a marina combined exhaust and inlet manifold with the hif38.
IslipMinor
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Re: Help With Engine Rebuild/Larger Head

Post by IslipMinor »

2 points on your proposed engine spec:

Rockers - will the standard rockers line up with the valves correctly in a 12G940 head? If not the use the 1275 rockers.

Exhaust manifold - both 1.75" and 2" are too big for a 1098 engine. The Maniflow manifold is listed as 1.625" OD, which will suit a modified 1098 very well.

On the point of using the choke, many of the alloy inlet manifolds are water heated, which definitely helps with driving during the 'warm up' period.

What cam are you going to use? The MG Metro would be a very good starting point.

Otherwise sounds like a very nice engine coming along!
Richard


MarkMan900
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Re: Help With Engine Rebuild/Larger Head

Post by MarkMan900 »

Thanks for all the advice folks! Glad to hear it's got legs as a plan.

- Is recessing the exhaust valve a better option that machining the main block? The chap I'm using refurbishes heads to order so can hopefully do it at the same time - or is machining the block better?

- I didn't know the rockers wouldn't fit but that's no problem to get more. Incidentally there seem to be a lot more 1275 A+ rockers about than standard 1275 A ones, I presume these are a no go?

- Is the 1.75" exhaust being slightly too big a problem that will cause less power or just potential overkill? I currently have very good stainless 1.25" standard exhaust but was under the impression this is too restrictive?

- Luckily I already have a brass tap, presumably the last person to own the car did away with the push valve when she had work done at some point.

- Cam wise I was going to stick with my standard one (assuming it isn't shot to pieces like the pistons and valves!). Is this a mistake?

Sorry for more questions but I'm a bit of a novice when it comes to modifying or rebuilding engines! Thanks again.

Mark
paul 300358
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Re: Help With Engine Rebuild/Larger Head

Post by paul 300358 »

My exhaust is 1.75 and it goes brilliantly. I asked to have the valves recessed when I had the hardened seats fitted.
philthehill
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Re: Help With Engine Rebuild/Larger Head

Post by philthehill »

Getting over the problem of the exhaust valve fouling the block can be done either by recessing the exhaust valve or machining a section out of the top of the block (see below).
Both have their set backs - recessing the valves can and in most cases upset the valve train geometry which can lead to additional wear of the valve train. Machining a section out of the top of the block if not done carefully and with insufficient land being left between the top of the top ring swept area and the cut out can lead to overheating and ultimately failure of the top piston ring(s).
I personally am not a fan of recessing the exhaust valves and prefer to machine the top of the 948cc/1098cc block for 940 head exhaust valve clearance.
If the valve is recessed into the head a shim of thickness equal to the recession should be fitted under the valve spring to keep the spring rate the same.
1100cc block with 940 cut outs.jpg
1100cc block with 940 cut outs.jpg (87.84 KiB) Viewed 2498 times
The MG Metro cam is a good choice (identifier below). The standard cam will give good torque but limits the benefits of fitting the 940 head.
MG camshaft 2.jpg
MG camshaft 2.jpg (169.75 KiB) Viewed 2498 times
The 1.625 O.D. exhaust is more suitable than any larger size. Larger pipe internals can slow the gasses down and reduce power.

I would recommend that you fit a duplex timing chain set up with lip seal timing cover and a crankshaft damper as the 1098cc engine is not the smoothest of engines.

Declan_Burns
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Re: Help With Engine Rebuild/Larger Head

Post by Declan_Burns »

Before you decide on whether to pocket the block (which is not reversible), or sink the valves, measure the distance from the bottom of the valve to the plane of the cylinder head with a vernier. You may not have to do either. 8mm is a safe distance. On my cylinder head it was not far off and I used a valve seat cutter.
You will also need a different thermostat housing. David Manners has them but at a price. Some use a bottom hose instead. You also may need a longer accelerator cable.
I used the 1098 pressed steel rockers and spacers but I had to turn a recess in some of the pillars for alignment. That was 10 years ago and no issues.
Regards
Declan


Regards
Declan
philthehill
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Re: Help With Engine Rebuild/Larger Head

Post by philthehill »

If you have a 940 head which has already had hardened valve seats fitted you will not be able to cut them - you will have to have them ground. The seats are too hard to allow a normal valve seat cutter to function.

paul 300358
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Re: Help With Engine Rebuild/Larger Head

Post by paul 300358 »

Well remembered Declan. I fitted an MGB mk1 thermostat housing with the holes slightly enlarged. Throttle cable came from eBay.
Declan_Burns
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Re: Help With Engine Rebuild/Larger Head

Post by Declan_Burns »

philthehill wrote: Wed Feb 19, 2020 8:24 am If you have a 940 head which has already had hardened valve seats fitted you will not be able to cut them - you will have to have them ground. The seats are too hard to allow a normal valve seat cutter to function.
Very true-measure it first and then decide.
Regards
Declan


Regards
Declan
panky
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Re: Help With Engine Rebuild/Larger Head

Post by panky »

Just to throw another (controversial) idea out there I found I needed around .020" to get the valve to block clearance measured without a head gasket. Looking at the exhaust valves I noticed the faces of the valves had loads of meat on them (for heat dispersal I believe.) so I actually reduced the thickness enough to get the clearance.
This is a valve after skimming, you can see there's still a lot if material left. When comparing the valves to those fitted in the stage three head on my Mini the ones I modified still have more metal in them after skimming
One other thing to remember is to use a 1275 head gasket.
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MarkMan900
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Re: Help With Engine Rebuild/Larger Head

Post by MarkMan900 »

Thank you all so much for the help - this has really helped clarify the plan and what options I have!

Regarding the duplex timing chain that's no problem, I have a big order to put in with ESM anyway so will grab it from them, I definitely want this to be a good long lasting engine as the car is my daily driver.

I've never really come across crankshaft dampers before, a quick google suggest the more tuned the engine is the more important it is. Is there a fairly standard/mild one that people would recommend - I have seen one on minispares but not sure if that is the best option?

I'm also considering this - https://www.ebay.co.uk/itm/MINI-COOPER- ... %7Ciid%3A1

Are big foot rockers ok or a mistake - it's mainly that they seem pretty reasonably priced so if they work then why not!

Thanks

Mark
paul 300358
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Re: Help With Engine Rebuild/Larger Head

Post by paul 300358 »

This is the type fitted to mine, https://www.ebay.co.uk/itm/Classic-Mini ... Swn1Jd36xv They are designed to fit all the a series engines. Be careful of fitting higher lift rockers or a high lift cam as this will require more material to be removed to pocket the block or sink the valves or skim the valves (whatever you chose)'
philthehill
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Re: Help With Engine Rebuild/Larger Head

Post by philthehill »

This is the type of damper you should fit to the 1098cc engine:-

http://www.minispares.com/product/Class ... o%20search

Avoid S/H units unless you have looked closely at the rubber. If cracked or perished avoid as the damper can fly apart.

The engine does not have to be tuned to gain benefit from fitting the damper. The type of damper in the link above was fitted to later BL standard engines as a matter of course.

The big foot rockers were fitted to the Cooper S but they are not the ones shown in the 'e' bay link.

This is a big foot rocker - note the wider pad which rubs against the top of the valve stem:-
Cooper S rocker.jpg
Cooper S rocker.jpg (172.7 KiB) Viewed 2373 times
You need to make sure that the pad is not pocketed. If it is pocketed avoid.

panky
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Re: Help With Engine Rebuild/Larger Head

Post by panky »

An unfortunate consequence of fitting a damper is that you loose the timing mark notch on the pulley as it's in a different place on the Mini version. So line up the timing marks with the original pulley, fit the damper, and transfer the mark onto it. A shallow hacksaw cut or chisel mark should be enough.
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Declan_Burns
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Re: Help With Engine Rebuild/Larger Head

Post by Declan_Burns »

Exactly, and you can add a scale. I still have to add the mark on the damper.
Regards
Declan
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Timing marks 2.jpg
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Declan
MarkMan900
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Re: Help With Engine Rebuild/Larger Head

Post by MarkMan900 »

Perfect. Thank you all so much! Finally starting to understand a little more about this and what it takes to build an improved but still reliable engine.

I noticed above you mentioned about needing a new thermostat housing - I had a look on ESM and they just sell one housing for all OHV engines so does that mean it won't fit the 1275 head as well?

Thanks

Mark
Declan_Burns
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Re: Help With Engine Rebuild/Larger Head

Post by Declan_Burns »

Mark,
The standard Morris thermostat housing has the wrong angle and the wrong height. I installed a sandwich plate under the David Manners housing to match the height.
You need this one.
https://www.morrisminorcentrebham.co.uk ... 5cc/12m220
Regards
Declan


Regards
Declan
philthehill
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Re: Help With Engine Rebuild/Larger Head

Post by philthehill »

I suspect that this is the sandwich plate Declan is referring to:-

http://www.minispares.com/product/Class ... o%20search

I have had to use the same sandwich plate on my Minor fitted with the 940 head.
The side outlet can be tapped and blocked with a brass plug.
100_3406.JPG
100_3406.JPG (1.42 MiB) Viewed 2297 times
I used three rocker cover studs (smothered in Copper grease) instead of the special BL screws.

Declan_Burns
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Re: Help With Engine Rebuild/Larger Head

Post by Declan_Burns »

That's the one Phil. I got it with the studs from the late bmcecosse RIP.
Regards
Declan


Regards
Declan
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