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TR5/TR250 Head Skimmin' TR250/6

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glemon

glemon

Yoda
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I have the motor out of my 250, and should have the head off tommorrow night, I am going to skim the head to raise c.r. a bit, I want to increase performance but not compromise drivability/reliability, at a little over 100hp stock and 2.5 liters their should be some room to do this.

So anyway from what I have read looks like rasing compression and a mild cam will help.

How much needs to be skimmed of to raise the compression ratio to 9.5 or 10 to 1 --is that a good target for the type of motor I am shooting for?

Thanks, I assume the 250 head is the same as early TR6, also not knowing the history of the car, what is the best way to tell if the head has already been skimmed?

Thanks, Greg
 
Glad to be of service. Good luck and post back with what you did and how much you took off.
 
You need to CC the combustion chambers, seat height alone can vary combustion chamber volume. While it great to have a nice galss burett with a stnd, you can do with much cheaper tools. Go to a drug store and get a feeding tube syringe, get a piece of plexiglass large enough to cover the combustion chamber, drill a hole in it to add fluid. Use a fluid ( I use rubbing achohol ). level the head with the combustion cahmber pointing up ward, use your piece of plexiglass, seal it to a combustion chamber will a little grease for seal, make sure you have a sprk plug in, then with your filled feeding tube syringe, record how much liquid it hold, don't worry MLs is the same a CCs, record the volume of each combustion chamber, then use one of the online compression caculator to see what CR you have, the guess work is how much to cut off, but hoepfully the fore mentioned link will help you there, but you need to know what you have first.
Hope this helps. I would lean towards 9.5.
 
Hap,

I agree and I thought that I said the same:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]How much is based on what you're starting with.

I took .120" off of mine to get to 9.5 to 1 verified by CC'ing the head. [/QUOTE]
 
I gave my machine shop the Goodparts information then told them I wanted 9.8:1 for compression when it was finished and definitely to stay under 10:1. Then let them worry about it. Because of the irregular taper of the combustion chamber the head was cc'd more than once to accurately get where I wanted.

Why under 10:1? It was a personal decision I was comfortable with based on reading threads on forums and advice from some that went over that level.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]It was a personal decision[/QUOTE]

And a rather wise one with gasoline being what it is today.
 
I am leaning in the 9.5 to 9.75 to one area, I just pulled the engine this weekend and started stripping last night, including pulling the head.

I am very pleased with what I have seen so far, bores are clean no discernable piston ridge at the top, very little carbon build up.

Crank end float just at .01" so slightly out of spec, but not the dropped the thrust washers chewed up the block/crank sort of thing I was worried about.

Water pump looked good, lifters slighly worn, but I was going to get a cam anyway so will have to do the lifters as well.

Do all hotter cams for these engines needed upgraded springs, or just the hotter ones?

Is this an engine that responds well to head port work?
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Do all hotter cams for these engines needed upgraded springs, or just the hotter ones?[/QUOTE]

This could be a topic all on it's own.

I would suggest that you follow the recommendation of the company that sells you the cam & lifters.
 
Brosky said:
I would suggest that you follow the recommendation of the company that sells you the cam & lifters.
Click to expand...
I'll ditto that.
But add that in my experience only some 'hotter' cams also need changes in springs, etc. For example, I believe the TR5 used the same valve springs as the TR250 even though it had a considerably more aggressive cam.
 
THe Goodparts link looks like it starts with the later lower compression motor, is the overall head thickness the same for the later and earlier motors, I am supposed to be 8.5 to 1 stock, I am talking to the machine shop today.

Brosky/Paul, the .120 you took off, was this on an 8.5 to 1 (stock) motor) or a later one?

Thanks, Greg
 
Are you addressing the bottom end? If not, how old is its last re-build? You can easily wind up with a smoker/ mosquito killer by adding a tight top end and raising compression on a loose or worn bottom end. When you remove that much material from the head, you change valve train geometry. This can require shorter push rods in order to get proper geometry back. You also may force the requirement for super-unleaded fuel. Be sure and up-grade to hardened exhaust seats to offset unleaded fuel damage to older integral seats and prevent the seat from sinking into the head material. Good luck!
 
the whole motor is being specced out by the machine shop, hopefully as I type, I will do what is needed for a rebuild/refresh the motor. I am looking for some ballpark figures to double check whatever I and the shop come up with.

So far everything I measured, bore, bearings, are stock (and not very worn either) but I am haven't checked the head thickness yet, it is at the shop, but I will before we cut anything.

Greg
 
Got some good information from the Triumph Prep Manual, which says the 8.5 to 1 heads are 3.460" thick and the later head 3.550. Also learned the TR250 head has different intake spacing than the TR6.
 
Don’t' forget to do the cam bearings. You just can't put any old tall cam and big springs in that motor. It will eat into the casting. 1st decide on the cam to determine if you need the cam bearings installed. I have the GP2 and no cam bearings. Not needed with this cam. Now I want to step it up with a meaner cam and can't. The block will not take the wild stuff, you know the tall lumpy lobes and orbit pressure springs. Nope. No can do. Get the cam bearings. Some machine shops don't do it because they may not have the equipment or the patience. It will cost you. But you will thank me.
 
I agree with Al, but others feel it's overkill. It's really your call, but if you go above the GP2, you really should do it. Richard Good sells a lot of cam bearings for a reason.

Here is what you will need to have done if you choose this route. And remember, NO shortcuts allowed.

TR6 Cam Bearing Installation
 
Greg,

I took .100" off of mine to get it to (approx) 9.0 to 1 but didn't verify it by CC'ing the head.
I used a TH5 cam from BPNW which has more overlap than stock but only a little more lift, as my valves/springs were in good shape I didn't touch them.
I used the nitrided tappets with the drain hole from BPNW and shorter (100thou) pushrods to correct the geometery.
Overall 1,100 miles on, and it revs exceptionally well and really gives a lot more power sensed through the buttmeter.
I had some trouble getting the idle to behave but finally got it to an acceptable point by fixing the bypass diaphragms on the carbs.
As Sherman noted, I made sure the bottom end was sound before adding the power by changing main, rod and thrust bearings, I also replaced the rings.
Overall a good way to make power without breaking the bank.
 
Greg,

Along with what was machined from my head and verified by cc'ing, I also had .008" removed from the deck of the block. Everything combined gave me a 9.5 to 1 compression. My point in telling you this is that the head thickness is critical, but machining the deck for zero deck clearance can also raise compression.
 
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