Rebuliding 1974 2.5L, N. American version.

[Rabid_Dolphin]

Well, it looks like I will be going into the block this Christmas as well /ubbthreads/images/graemlins/grin.gif . I will be installing new rod bearings, rods, pistons, and having a little head work done. Hopefully, we'll be able to purchase the pistons and rods preassembled and ringed. The head has been milled before, and my Dad is looking for the documentation to provide us with a better idea of the current status. Are there any varients of the stock valvetrain components worth looking into at this point? Would there be any longevity issues associated with aluminum pistons? Are there any particular head gaskets we should avoid? Does anyone know of an instructional/guide site dealing with this type of project? I have the Bentley manual, but some hands on tips would be great. Any help or recommendations would be greatly appreciated.
Andy

 

[Cain]

I had the small block rebuilt on my TR6 last year. My local LBC shop ordered news pistons. The +0.020 pistons came with the rings, but I don't know if they were separated or ringed. I used the original rods - just had the machine shop make them true.

I had the machine shop do the machine work and the LBC shop put in the crank, pistons, and cam - then I took over from there. I had never done anything like this before, and just walked myself through it with my Bentley manual, daily posts on this forum, and the guy at the LBC shop (I buy parts from him; he gives me information). So, regarding an instructional guide I don't know of one off-hand, but I can highly recommend coming here if you get stuck. Just give the specific problem and you are sure to get feedback with hands on tips.

I don't know much about mods to the engine - I just used stock parts - so I can't really help you on those issues.

 

[MDCanaday]

The head needs to have a lot of metal taken off to even get to 9/1 comp. 125thou was added to "federalise " the motor. You must remember to get shorter push rods when doing this so as to not upset the rocker geometry......
I am a big fan of forged steel rods, stronger and lighter can only help the high end......and the power made there!!!
MD(mad dog)

 

[Rabid_Dolphin]

I believe the head has already been milled 40-50 thousandths. Good call on the pushrods. Any idea about the Al pistons?

 

[piman]

Hello RD,
what other pistons can you get apart from aluminium alloy ones?
By the way, MCCanaday, Kas Kastner says the stock Triumph connecting rods are suitable for race tuned big sixes, and not many people got more power than he did from those engines.

Alec

 

[Simon TR4a]

I think the piston question is about forged or cast, since they are all aluminium, some with significant amounts of silicon in the alloy.
The stronger forged pistons are good for very high compression engines but require larger clearance to cylinder wall, so noisy for street cars till they warm up.
Simon.

 

[Rabid_Dolphin]

Indeed, my question was about forged vs cast Al pistons. I'm not particularly interested in hearing piston slap for the first 10 minutes after cranking the engine. I doubt this engine is going to see more than 10:1 comp, so forged pistons won't be necesary unless Dad wants to spray it. /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/nonono.gif

 

[Yellowdog]

With regard to the valve train, Good Parts handles what appears to be a very nice set-up with needle bearings on the shaft and a roller bearing on the rocker tip. It also has a higher lift ration to provide a bit more opening time for the valves. IMHO it is kind of spendy ($700.00+/-) unless you are upgrading the cam, intake and exhaust.

 

[Rabid_Dolphin]

I'm not thrilled about the way the cam just rides in the block. 700 bucks for 12 rockers sounds like kind of a crappy deal to me, but I might be intersted in switching to a slightly more aggressive camshaft. Where are these heads weaker: intake or exhaust?

 

[healeys4me]

There are many directions you can take with these 2.5 L engines. My son and I did the following with his car:
Milled .90 off the head
New alloy pistons
Header
1.75 SU carbs
emmissions removed (74 model)
Free flow exhust
high lift steet cam (APT)
no need to add cam bearings
Improved air filters
Results - gobs of power, still has street manners, and a tad loud. . .

 

[Rabid_Dolphin]

As it sits, the head has been milled an undetermined amount, it has the 3 into 1 headers, emissions deleted, 1.75 Strombergs, exhaust that probably needs to be replaced, and racing air filters. Could you give me a little more info on the cam? Just more lift? Do the heads flow well enough to benefit from more lift?

 

[LastDeadLast]

Here's a link to a tech article that tells a little about the benefits of increasing the rocker arm ratio of the TR 2.5. He also sells cams and rocker arm assemblies.

https://www.goodparts.com/tech_docs/Roller_Rocker_Benefits.html

-Shannon

 

[healeys4me]

No problems with the head flow. The lift and duration are both up from factory specs. Look over APT's site, we have the fast road cam.
https://www.aptfast.com/

What ever you choose, it's fun stuff. Best of luck,

John

 

[Rabid_Dolphin]

Alright, having some cylinder head trouble. Almost all of the studs came loose while trying to remove the head. About 5 of the nuts came off properly, leaving 5 studs still running through the head. Now I can't get the thing to budge. What's the secret? I've never had this much trouble before.

 

[Rabid_Dolphin]

Alright, here are some pics of my progress.
Gotta start somewhere.

Int manifold came off easily.

Here's the head, complete with from the rope trick. It was stuck on the block so well that I slid the tires instead of turning the motor over when I pushed it /ubbthreads/images/graemlins/lol.gif

What do you think of these combustion chambers?

 

[Webb Sledge]

I'm no expert, but it looks like you might have a couple burnt intake valves on cylinders 2 and 3 (from left in pic). 'Course that could just be a bunch of carbon buildup, it's hard to tell.

 

[Rabid_Dolphin]

It really just looks like soot to me. I think that would be weird anyway, since 2 and 3 don't share an intake runner.

 

[Rabid_Dolphin]

Can anyone give me a baseline number for head height so I can figure out how much has been taken off this thing?

 

[Bugeye58]

This is from a post awhile back, submitted by Piman.

there are various head thicknesses, UK TR6's had a 3.400" (nominal) head, stamped 516816, TR6 carb and 250 had a 3.535" head stamped 218227 and 5163233 respectiveley and replacement head for America were 3.560 stamped 219019.
The 3.4 head usually gives a 9.5:1 compression on a standard 2.5 engine.
Copper head gaskets were used on early engines but most sets I have come across lately are steel composite. I would have no problems with a copper gasket, but would double check that all the holes line up.

Hopefully that will give you some useful data.
Jeff

 

[vrod]

the size of your cylinder bore if its been bored out to a larger size will change your compression a little. There is a chart on the good parts sight that lists compression ratios according to your cylinder head thickness and piston cylinder size.