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MG TD TF 1500 - Power output

What sort of power should I expect at the wheels on a 1250 TF (+ 0.020 bore). ?
I have 40 BHP at 3500 RPM and still slowly rising. I am happy with the performance but interested.
Ray TF 2884

PS, I have to be happy. that lump is never coming out again !!!!!
Ray Lee

Here's a fairly acurate representation of a std 1250 engine output

Standard spec is 54 BHP @ 5200 RPM

If you've got 40HP at the wheels at 3500 normally you'd add roughly 20% for drivetrain loss on the dyno giving you roughly 48 HP
I'd say the graph shows about 40HP at your quoted 3500 rpm , so you're looking a pretty healthy 8HP up at that point
Be aware though that dynos are user adjustable for air temp etc so not a completely accurate comparison with the chart----but your result's up enough for you to be more than happy I'd say

willy


William Revit

I was going to go up to 4500 but was put off by a friend on the way to have his done. His motor let go at 4500. The little end let go and the conrod flailed through the sump and oil pick up and baffle. Subdued us a bit,between us we will have enough bits to get him back on the road.
The bad thing is that his brand new billet crank is scored on number 3 journal.

Ray
Ray Lee

I have to say, that if your engine isn't safe at 4500RPM then I wouldn't drive the car. Stock redline is nearly 6000RPM. If it's a short sprint on a dyno then I would take it to 5000. Some would go higher.
Steve Simmons

Hi Steve,
I run a 3.9 diff on a standard gearbox. My cruising speed is 70mph / 3600rpm which means I am always in the sweet torque area. Nearly flat 2000/3000 dropping slightly 3000/3500 Newman fast road cam
I have been on this crank for 250,000 miles and want to keep it that way.
I know the crank is safe as I have often thrashed it with the original diff. That little niggle in the back of the brain is still there though.
Ray
Ray Lee

Sounds nasty Ray , do you know if it was a little end bolt failure or the rod itself let go--Dyno man wouldn't be happy I expect with MG all over his floor
William Revit

Good point Willy. Count me as another who would be interested to know which component let go. I once found a small crack in the small end of a rod. That rod was about to go into a race engine. I was sure glad I had a crack-testing kit handy. (Dye spray type.)
Willy, are you going to Winton in May? If so, contact me at bobschapel at optusnet dot com dot au
Cheers,
Bob
Bob Schapel

William,
Pinch bolt failure,the eye unwrapped. The engine was rebuilt last year and I don't know what kind of bolts he used. I will see the remains next week and pass on the info.
I will only use Allen head bolts as the original type are hard to get a socket on. We worked on an engine last year where DPO had held the con-rods in a vice to tighten the clamp!!!!! all had to be replaced.
The one that blew up was on the road going to the dyno. He is having no luck,two years ago his crank snapped hence the new billet crank.
Ray
Ray Lee

Cheers Ray
It'll be interesting to see the remains of the bolt, if it hasn't escaped , the markings on the head should indicate what the bolt is/was
As with you I've used Unbrako's, they just seem a natural good fit for the pinch bolt job

No Winton for me Bob ,I'm very covid shy, haven't travelled for a couple of years---and can't see it happening anytime soon---

willy
William Revit

Hi Ray,
Some interesting issues with small ends. I guess that whatever the bolt used in that failed small-end, the torque used to tighten it will never be known.

Firstly, the 33.3 ft lb torque listed somewhere is much too high for a standard bolt. The small end bolts are the same diameter and thread pitch as the con-rod bolts which are bordering on stretching at the 27.? that is listed for them. I have used original s/h small end bolts in ALL the XPAG motors I have assembled but have so-far been able to select bolts in good UNSTRETCHED condition from shed contents. (The only exception being my current race engine which runs an experimental solution .... Not described here as it falls into the "Don't try this at home" category.) However, I do admit that Unbreako allen head bolts do make more sense than using old fatigued bolts. I'll use them next time.

Secondly, I have never seen an "unknown" set of XPAG rods which did not need straightening! Maybe they are all bent or twisted because people tighten/check the small end with the big end on the crank or clamped in a vice like your DPO. Using a "pin" through the gudgeon is a good way to secure the piston during torqueing. Maybe machine shops are not as fussy as we are? I hate to think of the weird movements a piston must perform if the con-rod is bent and twisted. Con-rods are easy to straighten yourself. All you need is a nice straight piece of machined rod of gudgeon-pin size, a piece of square steel RHS and a good vernier caliper. I don't think con-rods usually lose "straightness" in service. I found my own road car con-rods perfectly ok after years of hard use when I last did rings and bearings (many years ago).

Thirdly, I think XPAG small ends rarely crack, but I have experienced it, as mentioned in my previous post. So I believe it is worth while to crack-test con-rods as well as cranks. Spray kits are effective for that.

Sorry I won't see you at Winton Willy. But I understand the situation. Lat's hope that the Covid situation improves.

Cheers,
Bob




Bob Schapel

I've never been a fan of the pinch bolt design on a rod and nearly every XPAG I have seen destroyed had to do with a failure of the small end of a rod. The last two XPAGs I assembled got a set of these.





L E D LaVerne

Yeah interesting the TD workshop book quotes 400 in/lb which is 33ft/lb and the S/P book has 27ft/lb for everything from TC through to MGB, and, MGB Special Tuning specs lists them at 25ft/lb

William Revit

L E D LaVern, could you give me the model or part number for those Saenz rods. I looked on line on the Saenz web site ,but could not find any information on Xpag engines.
R E J Stewart

WOW!!!!!!




Ray Lee

A disaster.
One big end bolt has stretched to a point and the threaded end is nowhere to be found. The other is bent around the con rod as in photo it's nut has been stripped off shearing the split pin on it's way.
It looks like one big end bolt failed, the cap then unwrapped. The con rod left the crank and was smashed against the bottom of the cylinder. This broke off the bottom of the cylinder and then snapped the conrod.
Various bits flailed around destroying the cam, dizzy and oil pump gears.
I have not seen inside the engine yet but I do not hold out much hope fore the new billet crank that is in it.
The engine was built by a member with many years of XPAG experience so I think it must have been a part failure. All new fasteners were used.
The photos were taken at our club/pub meeting last night and we think between us we have enough parts to get him back on the road.
Ray TF2884


Ray Lee

Oh sh%t, what a mess, Like you, I don't hold out much hope for the crank
It always amazes me how much damage can happen in such a short time.
I'd be interested to know where the bigend bolts came from so as not to get caught with a set of them---

willy

William Revit

He was doing about 3000 rpm /60 mph with his axle ratio at a steady speed when it blew.He had no warning whatsoever
Everybody forgets that this is 100 reversals a SECOND! the forces involved are immense.
I may see the crank this weekend and work out what went first.
Ray
Ray Lee

Ray, from innocent beginnings this has turned into a must-read thread for the detail and technical content/comments from some very experienced people. In my opinion this should be read & followed by all xpag (etc) owner operators. I live in an area where there is no local MG group, so am in awe that your club is able to help the poor guy stitch things back together.
Good luck!
John.
J P Hall

John,
I am lucky, I am in the Octagon MG car club and our local group has 18 XPAG owners all living within a 25 mile radius. I have had my car for 48 years, others longer. We have a great group collection of bits and pieces which we swap,loan and gift,nobody is off the road for long.
Between us we have the skills and knowledge to keep us all going for a long time.
Ray
Ray Lee

Found why it blew.
When slant drilling for the crank oilways you must
first use an endmill to start or the drill will drift off.
Number 3 big end to the centre main had been endmilled but not been drilled through. Nor had number 1 to the front main,
Number 3 seized on the crankpin snapping the conrod which threw bits at the scew gearing bending the oil pump shaft and cracking the oil pump housing and ruining the dizzy drive gear.
The most amazing thing is that numbers 1&3 bearings had only splash lubrication for 6 years and 1000's of miles, number 1 only had wear marks!!!.
I don't whose crank it is yet, but will not say anything until they have had a chance to respond.
We have most of the bits to build a new engine,just have to source a late oil pump.
Ray TF 2884
Ray Lee

I thought his original crank went 2 years ago he says it was 5/6 years ago, i should nave remembered it is over 2 years since covid and it was well before that, see youtube "XPAG Seal fit"
Ray
Ray Lee

Not an xpag but a 1938 VA tourer. I was out for a sunday drive and BANG CLATTER it let go.
Fortunately I shut it off in time as there was zero collateral damage.
No 2 piaton had split the top off and the wrist pin end was wide open.
There was zero damage to the cylinder walls and the rod stayed connected to the crank.
Jahn's Pistons made me a new set of pistons and with a rebuilt rod the car was back on the road.
My horseshoe is still in my pocket.
Sanders

Unbelievable that it went that long--
Watched the video and you can see the oil holes on the bigends, looks like they're cross drilled, so you're saying the hole from the mains to the journal holes wasn't drilled what a disaster, he should get a free crank for a stuffup like that---but in hindsight, any engine builder that doesn't check/clean all oil galleries--well um, even though it was new you have to check check check, don't trust anyone ever at their word--do it yourself, check check check
Lost count of how many shonky new MGB oil pumps I've caught
William Revit

William,
Yes the holes look as though they had all been drilled. On 1 & 3 only the starter milling had been done.
When I first looked at the oilway I thought it was
melted metal from the shells blocking it. A check with a borescope clearly shows the toolmarks.
There was more than one person involved in the build,maybe each assumed another had done the check.
Ray
Ray Lee

Yeah, probably best to put it down to bad luck and move on, It's easy to get caught assuming that new parts are correct as you would expect them to be, specially on something you've paid out big money for, I bet there'll be some serious looking at the next crank though, eh
William Revit

This thread was discussed between 07/04/2022 and 21/04/2022

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